Require python >= 2.6, use the json module.

* Set the minimum version of python to be 2.6, since we don't need to support
  anything older anymore.

* As the first of a series of related cleanups, drop our custom json
  module (since json is in the stdlib since 2.6).
Merge pull request #36 from craigcitro/imports

Require python >= 2.6, use the json module.