Joda Period generators

[noelmarkham]

Generators for Joda Period classes:

• Full-range generators for the different Period types (Years, Months, Weeks, Days, Hours, Minutes, Seconds)
• Full-range generator for a Period consisting of years, days, hours, minutes, seconds, milliseconds
• A generator method taking a DateTime and a Period, and this will generate a DateTime between the given DateTime and the offset given by the period. Note this is different from previous discussions (which was DateTime ± Period), as I believe this makes things a little clearer.

   DateTime
      ▼
<-----|------------------------------------------|------------->

      ~------------------------------------------~
                        ▲
                 Period (range)

Everything seems quite straightforward right now, but I'd love to get some early input on the code, as well as some fundamental things like:

• Package name: One idea could be to change the package to org.scalacheck.datetime or similar, copying other libraries such as scalacheck-shapeless.

1. Class names. I expect I'll add an ArbitraryDateTime sometime soon containing sensible default arbitraries.
2. Other generator methods. I was thinking of adding some "helper" generators which defer to genDateTimeWithinPeriod, by calculating the correct parameters, such as methods which take:

A period either side of a DateTime (as noted above):

                       DT
                       ▼
<-----|----------------|----------------|------------->

      ~----------------~----------------~
              ▲               ▲
              P               P

Two periods, so that the offset either side of the given DateTime can be different:

                       DT
                       ▼
<-----|----------------|-------|------------->

      ~----------------~-------~
              ▲            ▲
              P1           P2

Note these two suggestions are not completely logical as a period has a notion of being positive or negative, and there's no abstraction that represents just the magnitude of a period (what does it mean to have a negative period for the time before the given DateTime???), I'm half thinking of just including documentation as to how to do this, to keep the API clean and reducing confusion due to ambiguity.

Finally, just to mention this code so far is only for Joda's classes, I intend to update this to generate different classes from other libraries, such as Java 8's Date and Time API.

Closes #1

[raulraja]

LGTM! This is awesome. Should add references back to the ScalaCheck docs to point to the lib once is stable and we have a solid version.