Limits/Offsets in Stacky API

Most Stacky API calls now have limit/offset query arguments.
The default limit is 50 and the hard limit is 1000.
Filter on when field for events in StackyAPI
store image_type from nova notifications
Merge pull request #187 from TMaddox/upstream-stable-4

store image_type from nova notifications
Made owner in ImageUsage and ImageExists nullable since it is possible for the Glance image to have no owner specified.
Merge pull request #188 from anujm/RM2014

Made owner in ImageUsage and ImageExists nullable
Merge branch 'master' of git://github.com/rackerlabs/stacktach into stacky_refactor2

Conflicts:
	stacktach/stacky_server.py
Merge pull request #160 from ramielrowe/stacky_refactor2

Limits/Offsets and When Filter in Stacky API
merging with master