Updated to gradle plugin 0.9.1
Added DateUtils class
Added relevant constants from DateUtils

I did not add all of the constants from Android's DateUtils.  Some of them are
unnecessary when combined with Joda-Time and others have been deprecated long
ago.
Added formatElapsedTime() and tests
Added isToday() and tests
Added formatDateTime() with tests
Added formatDateRange() and tests
Updated README to point out DateUtils