Implement BYEASTER. Uses the Anonymous Gregorian algorithm

Algorithm from here:
http://en.wikipedia.org/wiki/Computus#Anonymous_Gregorian_algorithm

(Which is also the one used by the holidays gem:
https://github.com/alexdunae/holidays/blob/master/lib/holidays.rb#L217)
Use eastermask properly. Bugfix by using Date.UTC.
Merge pull request #25 from wombleton/byeaster

Implement BYEASTER. Uses the Anonymous Gregorian algorithm