New DBM key-value store.

Add a new key-value store implemented with the DBM module provided by the Python
standard library.  It is specially useful for local development environments of
projects that use the Redis key-value store but cannot install Redis, or do not
want to.
Incorporate the new DBM key-value store to the test suite
Document the new settings added for the DBM KVStore
Merge pull request #290 from etanol/master

New DBM based Key-Value Store
Merge branch 'master' of https://github.com/mariocesar/sorl-thumbnail