Update sys.path, plus minor corrections.

This PR addresses a couple of minor issues:
 * Correction of a typo in the ```About``` dialogue.
 * Update to the ```.gitignore``` list to exclude ```lib```, so that all ```lib/thea``` source code changes can be added.
 * Update the Python path such that a user can start Thea with ```python main.py``` without having to create a ```thea.pth``` or set the ```PYTHONPATH``` within their environment.