Added support for LocalDate - required for serializing clj-time/today objects.

Hello, 

i added additional protocol for *LocalDate* objects. Now it's possible to serialize values created with *clj-time.core/today*, *clj-plus* or *clj-minus*.

For new protocol i used  AbstractPartial type,  [parent class of LocalDate](http://joda-time.sourceforge.net/api-release/org/joda/time/LocalDate.html) , because i noticed that protocol for DateTime also uses parent abstract class.

Currently,
it's possible to use *clj-time.coerce/to-date-time* function to coerce LocalDate object into DateTime.
But i found it's too tedious to convert those values all the time manually in my project, when it's possible to make my life easier by copy-pasting additional 3lines to monger.joda-time namespace.

Tests are in ns `monger.test.clj-time-local-date-test`.