Persist

* New persist method added to keys.js and the public interface
* New tests based on the tests for EXPIRE and the documentation for PERSIST at http://redis.io/commands/persist

The functional changes and tests are all in the first commit. The second commit is only a stylistic change and might not be to your taste. The tests pass after each commit, so you can take just the first one or both of them.

Cheers,
Harry