WIP: Move from Datetime to Dates

Ok, sorry for the hiccup earlier (where I auto-pushed these changes). 

Feel free to take a look and let me know if this works for you. Note that we need to hold off on merging until Dates.jl gets officially registered since I don't think you can have a registered package depend on an unregistered package.

All the tests seem to pass for me (though I'm seeing a weird segfault with the FactCheck package).