Adjust handling of failing cache sets

Sometimes the cache 'set' call simply fails and doesn't add an item to the
cache.  This changes alters the test for failure by simply checking if
anything is correctly set.  If nothing, then a RuntimeError is raised.
Catch exceptions raised when triggering task

To avoid 'connection refused' errors percolating up to the top.
Polish docs
Contemporise sandbox install

Was still using API from many months ago.
Adjust import of task decorator for backwards compatibility

Also remove app from runtests settings to allow tests to run with Celery
2.5.4.
Relax django-celery constraint so package works Celery 2.5

Fixes #4
Fix tuple unpack error when the cache is empty and fetch_on_miss is enabled

Fixes 8
Add invalidate method

Thi allows an cached item to be invalidated and asynchronously refreshed
without having to delete it completely.

Fixes #9
Document core API calls (get/invalidate)
Document refresh timeout setting
Group public methods together.  For clarity.
Added logging.basicConfig() in base.py

This prevents 'No handlers could be found for logger "cacheback"' errors in web server logs (tested with Apache).
Merge pull request #13 from osgl/master

Fix for 'No handlers could be found for logger "cacheback"' errors in web server logs
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