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:

1. 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.

2. 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