Implementing SCAN commands

Hi there,

First, thanks for maintaining tornado-redis!  This pull request includes two commits.  Just a couple of notes on each one:

* Adding SCAN commands

The unit tests are not very DRY, but I thought this was OK because generally I like to favor readability over DRYness in tests.  But please let me know if you think otherwise.

* Normalizing whitespace

My editor wanted to remove trailing whitespace in the recent changes for locking.  I thought this commit would be useful since the rest of the project seems to have no trailing whitespace.

Cheers,
Taylor