Remove elpy-enable defcustom.

It's apparently impossible to use customize to call a function in
your package to enable something. If the setter function calls
another autoloaded function, there's a load loop error.

Meh.
New test infrastructure.

Elpy can now use different test backends and does not require
nosetests anymore. In addition, the C-c C-t command will now
choose to run the test at point if there is any, otherwise
run all tests in the project.

This implements #140 and #181.