Docs and Bugfixes

This release improves the documentation and fixes a few bugs:

1. Cadnano2 now exits gracefully, rather than just closing the window but leaving the python process running.
2. Prompt user to save any unsaved changes before quitting.
3. Fixes a crash when launching the Paint tool color picker
4. We now suppress some console warnings that happen when the PyQt main window is created.

Full Changelog: v2.4.2...v2.4.3