Remove all % formatting once the Python 2.5 App Engine runtime is deprecated

[dhermes]

Since .format was not added to the str class until Python 2.6, we need to use nasty % string formatting, which is prone to errors.

As announced on March 8, 2013, the Python 2.5 runtime will be deprecated in January 2014. At this point, all the error-prone %-formatting can be removed and we can just use str.format. Happy, happy days.