add health check for storage backends

* checks a given storage backend by writing, reading and deleting a file
* extendable to check any StorageBackend
Merge pull request #2 from stefanfoulis/feature/storage-check

add health check for storage backends
Improved celery backend
Bumped version
Seems it works with django 1.2 as well
Version bump
Update setup.py
Version bump
Fix indentation
exclude build subdirectory, *~ and *#
Merge branch 'bugfix/indentation-error'
Version bump
.identifier is a function and should be called.
More documentation
Case study

from the world most interesting man
Corrected typo in README
Changed default path for file backend, the other would create a large number of directories
Added health_check.utils.healthcheck

Added a decorator that makes it a lot easier to write checks. Inside the
decorator you can raise health_check.utils.HealthCheckFailed or
health_check.utils.HealthCheckUnavailable instead of returning
status values.
The decorator optionally accepts a string with a descriptive name of the
check.
Refactored the health checks to raise exceptions instead of returning integers. This is backwards compatible and should not break anything.
Updated docs to show the new exception-based api.