elpy-enable: Check for python-mode.el

Elpy does not work with python-mode.el, and that can cause quite some
confusion due to various conflicts. Check whether python-mode is from
python-mode.el, and if so, throw an error.

Fixes #95.
elpy-initialize-variables: Reload yasnippets to ensure they are available.

Yasnippet does not do this automatically, and when the user hasn't
configured them, no snippets are available otherwise.

Fixes #94.
elpy-initialize-variables: Add the Elpy snippet directory.

Elpy now ships with a few YASnippet snippets for Python.

Fixes #43.
elpy-use-ipython/cpython: Make the command configurable.

The two functions now accept an argument for which interpreter to use.
Interactively, they prompt for it with a prefix argument.

Fixes #83.