Add support for Joda YearMonth

Hi,

Thanks for an excellent library!

This pull request adds support for the Joda YearMonth type, useful for things only requiring a year and a month, for example credit card expiry dates.

Implements:
- a (useful) subset of the DateTimeProtocol for the YearMonth type
- coercion to DateTime, using date-time constructor fn
- construction by year-month function

Please let me know if this is not a good fit for the clj-time abstractions or if there is anything I can do to improve it.