Cleaning up some trailing whitespace
Changed Distribution._sign_string over to use rsa

rsa is a pure-python alternative to M2Crypto when it comes to cloudfront's usage in _sign_string
Unconditional 'else' was causing breakage

While writing unit tests to make sure _sign_string works in all possible cases
I discovered that my unconditional 'else' clause was making it so that
private_key_string would never be used. That has been addressed.
Removed overeager seek

The .seek(0) in Distribution._sign_string isn't necessary, any preparation of
the file object that needs to be done can be done outside of this bit of code
Merge branch 'seandst-develop' into develop

This replaces M2Crypto with rsa to sign urls
in CloudFront, fixes #1214.

* seandst-develop:
  Removed overeager seek
  Unconditional 'else' was causing breakage
  Changed Distribution._sign_string over to use rsa
  Cleaning up some trailing whitespace