added last_update functionality

Changed the pygithub api so that the Repository.get_pull_requests()
function takes a "since" argument, and in this case, instead of the
PaginatedList that the method ordinarily returns, returns an actual
list that contains all pull requests from the given repo that are
before the "since" datetime input.
made main.py only reference the one handler Get
Fixed an ndb key/entity mix-up
utility function to update everything
GET_FOR[purpose] dict of funcs are uniform
pass make_task time to wait instead of datetime
removed multiple-pickling problems
docstrings weren't necessary, caused clutter
main has an Updater requesthandler now
LastUpdate should be autoupdated when put()
Actually record language in comments
users property is a list, so set repeated=True
actually recognize exceptions
fix unicode crap
neater code for _last_update return statement
remove initial delay from tasks
only ask for forks if there are any.
changed the way last-update works.
fixed some unicode/string issues
comments unique in datastore by github unique id