use the new style of string formatting

- Using the % to format the strings is in the process of being
  deprecated. It's recommended that the new style (defined by
  PEP #3101[1]) be used for new code moving forward.

[1] http://docs.python.org/3.0/whatsnew/2.6.html#pep-3101
Merge pull request #1 from theckman/new-format

use the new style of string formatting