Support for mocking calls with keyword arguments

[rofinn]

The following throws an error because the @mock macro tries to put all expr.args[2:end] into a tuple (or doesn't correctly distinguish args and kwargs).

Example)

@mock pipeline(`ls -al`, stderr=DevNull)

throws the following error

julia --compilecache=no -e 'Pkg.test("MyPkg")'
INFO: Testing MyPkg
MyPkg: Error During Test
  Got an exception of type LoadError outside of a @test
  LoadError: function tuple does not accept keyword arguments
   in kwfunc(::Any) at ./boot.jl:236
   in macro expansion at /Users/rory/.playground/share/test/packages/v0.5/Mocking/src/Mocking.jl:191 [inlined]
...