Move loaders/couchdb to couchdb

It doesn't look likely that other loaders will be developed, no need for a full sub-package here.
New mapping object used for models and sessions
New model classes
Remove model subpackage
Update base RequestHandler, update and move SessionRequestHandler

- Streamline session request handler and objects
- Make sure RequestHandler properly handles the ALLOW attribute and sends human readable JSON when using curl
Streamline sessions, remove sub package, make into module

- Now with Asynchronous redis support
Session cleanup and bug fix for delete
New tinman.handlers.mixins module

- Moved tinman.handlers.redis_handlers.AsynchronousRedisRequestHandler to tinman.handlers.mixins.RedisMixin
- No more synchronous redis support
- Added ModelAPIMixin and RedisModelAPIMixin
Additional configuration keywords
Minor import differences
Updated README, setup.py to reflect changes