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.
Assorted import/lint cleanup.

This is mostly a cleanup around import statements and a few related pylint
issues.
Merge pull request #37 from craigcitro/imports2

More import/lint cleanup.
Drop uritemplate.

The `uritemplate` module was only used in one place, for what amounted mostly
to a string replace, so we can simply drop it. Unfortunately the `META`
variable in that file is part of the public interface, so we need to keep it
around as-is.
Merge pull request #38 from craigcitro/imports3

Drop uritemplate.