Bump version to 0.4
Add release target to makefile
Switch to Celery 3.0's task logger
Add git-core to puppet manifest
Handle case where synchronous failover refresh fails
Tidy requirements
Allow cache write verification to be disabled

This is required when testing with the DummyCache which can't write by
definition.  Set CACHEBACK_VERIFY_CACHE_WRITE=False during development
to allow DummyCache to be usable.

Fixes #11
Improve docs slightly