Add limit support and tests.  Add order API, tests, and in-memory backend, but no client or Cassandra support.

Summary:
Add untested limit clause

Fix merge conflict

Reverse some accidental ordering functionality

Limit tests

Add order to API, tests, and in-memory backend.  No support in client or Cassandra backend.

Reviewers: usmanm, meelap

Reviewed By: meelap

Differential Revision: http://phabricator.locu.com/D2967