Move functions from quartzite.date-time over

I submit this as a pull request for review by @seancorfield and @alexbaranosky.

This moves several utility date/time functions from [Quartzite](http://clojurequartz.info).

I updated Joda Time and Clojure to the most recent stable versions along the way and
bumped the version to `0.5.0-SNAPSHOT`. Both upgrades are unrelated but seem like
a natural thing to do for `0.5.0`.