README: Remove pyflakes+pep8 recommendation.
elpy-project-root: Check that elpy-project-root is set.

The function could raise an error when the user C-g'd out of setting a
project root, as `file-directory-p` does not handle nil as an
argument.

Fixes #73
jedi backend: Add project root to sys.path for Jedi calls.

Jedi does not find definitions in packages when it can not import
those packages. Temporarily adding the project root to `sys.path`
allows Jedi to find these definitions.

Fixes #74