elpy-project-find-root: Return current directory if not in library path.

This is used by nosetests to run on the current directory if we're not
in some kind of library tree. To get a default project root, we pass
an argument to skip the current directory default.
elpy-rgrep-symbol: If not on a symbol, ask the user what to search for.

This used to search or "nil", which was not quite what we wanted.
README.md: Add a note about using the development version of elpy.

This causes issues with people trying to install the elpy version
from git but then get the outdated backend from elpy.

Fixes #27