Joda time integration. Issue #5. Previous commits handle issues #35, #36.

Created dependency on Joda Time. Provided integration with Joda time via
2 functions in AbstractDateModel: setLocalDate() and getLocalDate().
Note that the joda time LocalDate type is the joda type that most
closely resembles the existing JDatePicker get and set date
functionality. See joda LocalDate class docs for more details.