Implements the layout module.

This module combines the old 'navigation' and 'header-panel' modules into a single, configurable component.

- Added new layout module, with demo.
- Rewrote material-styleguide.html and index.html to use the layout module.
- Removed 'navigation' and 'header-panel'.
- Added README.md explaining usage of the module. We'll need a better place for these docs at some point.