Support parse when start dates are in the past

Prior to this change, Tickle would throw an error if the start
option passed in was in the past with respect to `Date.today`.
When attempting to enumerate intervals of dates that have already
occurred (in the case of looking backwards in a calendar), it is
important to be able to parse Tickle expressions and determine
what the next occurrence _was_ as opposed to when it _will be_.
While it appears that the parse API already supports a `:now` option,
it seems this option is ignored when checking this condition and,
instead, a `Date.today` is used.  This change simply uses the `:now`
as is passed to the `Tickled` object to validate `start_date`. Since
`Tickled#now` defaults to `Time.now`, this change is minimal but does
add value to the gem.

References: #4

lib/tickle/tickle.rb

@@ -50,7 +50,7 @@ def _parse( tickled )
       # check to see if this event starts some other time and reset now
       scan_expression! tickled
 
-      fail(InvalidDateExpression, "the start date (#{@start.to_date}) cannot occur in the past for a future event") if @start && @start.to_date < Date.today
+      fail(InvalidDateExpression, "the start date (#{@start.to_date}) cannot occur in the past for a future event") if @start && @start.to_date < tickled.now.to_date
       fail(InvalidDateExpression, "the start date (#{@start.to_date}) cannot occur after the end date") if @until && @start.to_date > @until.to_date
 
       # no need to guess at expression if the start_date is in the future
@@ -75,7 +75,7 @@ def _parse( tickled )
 
         # if we can't guess it maybe chronic can
         _guess = guess(@tokens, @start)
-        best_guess = _guess || chronic_parse(tickled.event) # TODO fix this call 
+        best_guess = _guess || chronic_parse(tickled.event) # TODO fix this call
       end
 
       fail(InvalidDateExpression, "the next occurrence takes place after the end date specified") if @until && (best_guess.to_date > @until.to_date)
@@ -131,7 +131,7 @@ def scan_expression!(tickled)
           fail(InvalidDateExpression,"the ending date expression \"#{tickled.ending}\" could not be interpretted")
         end
       else
-        @until = 
+        @until =
           if  tickled.starting && !tickled.starting.to_s.blank?
             if tickled.until && !tickled.until.to_s.blank?
               if tickled.until.to_time > @start

spec/tickle_spec.rb

@@ -241,6 +241,14 @@ module Tickle # for convenience
         end
       end
 
+      context "Given that start is in the past, respect now option in parse" do
+        context "every other day" do
+          subject{ Tickle.parse('every other day', {:start=>Time.parse("2009-05-09 00:00:00 -4000"), :now=>Time.parse("2009-05-09 00:00:00 -4000"), :until=>Time.parse("2017-10-21 00:00:00 -4000") }) }
+          let(:expected) { {:next=>Time.parse("2009-05-11 00:00:00 -4000"), :expression=>"every other day", :starting=>Time.parse("2009-05-09 00:00:00 -4000"), :until=>nil} }
+          it { should == expected }
+        end
+      end
+
       context "Given that now is in the future, 2020-04-01 00:00:00 +0000" do
         context "February" do
           subject{ Tickle.parse('February', {:start=>Time.parse("2020-04-01 00:00:00 +0000"), :now=>Time.parse("2020-04-01 00:00:00 +0000")}) }