Support for basic OpenStack API operations and Keystone authentication

Fixes #373 #374

* Add rewrite rules for basic bucket and object operations from
  OpenStack Object Storage API
* Add authentication module for Keystone authentication service
* Make URL rewrite module configurable
* Refactoring to further decouple API requests and responses from core
  object store functionality.
* Add helper module for JSON processing
* Refactor XML handling code
* Isolate configuration functions into riak_cs_config module