Improved instructions to install and run tests in README.
Fixed #20 -- TextInput is now correctly deleted.

Also fixed issue on FocusMixin not being correctly deleted.

Thanks to @baccenfutter for reporting the issues.
Merge branch 'issue_20' of https://github.com/jorgecarleitao/pyglet-gui into issue_20

Conflicts:
	pyglet_gui/text_input.py