Customization rework.

A number of variables are now customizeable, some of them only
to make setting dir-local values easier. This is used by the new
function elpy-set-project-variable, which sets dir-local variables
in the .dir-locals.el file in the project root.

Also, flymake-forward/backward-error are now called flymake-next/
previous-error.
Sphinx documentation for Elpy.