Fix a typo in a symbol name.

One of the global constants in client.py had a typo (`VERIFICATON` instead of
`VERIFICATION`); it's hard to *fix* without (potentially) silently breaking
people, and it's not easy to provide a deprecation warning on a module global.
For now we simply bind both names.
Update except statements to use `as` everywhere.
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.