Model take reactor as argument

instead of directly importing. This gives better control while testing
and is good practice in Twisted community
health clock is explicity set
unitgration tests didn't have reactor
Merge branch 'master' into store_has_reactor

Conflicts:
	otter/tap/api.py

Conflict occurred since mock store was removed and store was created
with reactor in this branch. So, removed mock code and created store
with reactor
health_check does not take clock - thanks @radix

instead uses self.reactor stored in its object.
Merge pull request #593 from rackerlabs/store_has_reactor

Store has reactor