Use gethash, not getenv, to access config.

Fixes #393.
Do not throw an error if Flymake is not loaded.

Users not activating the flymake module could experience errors
when flymake was not loaded. Prevent these errors.

Fixes #394.
elpy-use-cpython: Set interpreter to correct value.

This now uses the argument string in Emacs 24.4, too, instead of
hardcoding it to "python".

Fixes #392.