started update_triggers
coded. yet to test
schedule datetime object change based on silverberg 0.1.2 change

- storing datetime object instead of iso string in cass since silverberg supports datetime marshalling in
0.1.2.
- added method in mock to pass tests
- updated tests accordingly
Merge branch 'master' into update_triggers
lint and str fix
storing far future date for 'cron' schedule
Merge pull request #237 from rackerlabs/update_triggers

update_event_triggers model API

Model API to update trigger time of given policy ids
Passing datetime object instead of iso string to silverberge since it supports datetime marshalling in 0.1.2.