Updated demo to use startup method on an App class.
Updated demo to use new split package structure.
Modify order of content assignment significant.

This is needed as a temporary fix, primarily for the GTK backend.
If the toolbar is created/assigned after the main content, it isn't
created as part of the window. Long term, order of creation shouldn't
matter.
Corrected markup in README.
More markup fixes for README.
Bumped version number for release.
Modified setup.py to use setuptools platform detection rather than if statements.
Back to development mode.
Simplified end-user dependency to be just 'toga'.
Bumped version number for v0.1.2 release.
Bumped version number for v0.1.3 development.