findundefined, autoimport: Do not add duplicate modules.

There was a bug that added a module multiple times if it was used
multiple times in the same source file. This commit should fix that.
elpy-tests.el, test.sh: New files to enable automated testing for Lisp.

Elpy now has some unit tests for the lisp code. The coverage is not nearly
as extensive as the Python tests are, but it's a start. The test.sh
script can run elpy tests in a number of different Python versions.
elpy.el: Fix some docstrings, rename some functions to internal names.
build.sh: Also install elpy-refactor.el.