Add completion test for virtualenv site-packages.
Use test fixtures in ert tests.

Remove hardcoded python sources in tests definitions.
Add company-jedi-stop command.
Add company-jedi-install command.
Introduce company-jedi-mode.

Manual change for python-mode-map is unnecessary now.
Add lisp equivalent for each start_jedi cli option.

Build `company-jedi-command' interactively.