Remove card demo from shadows

This is no longer needed and adds confusion
Move paper-card into shadow demo
Add new cards component implementation

This introduces a card component that uses the existing shadows for
depth.
Merge pull request #459 from sgomes/material-sprint

Switching header panel header text to correct typography
Improving navigation.
- Added content to all examples, to illustrate usage.
- Added "centered horizontal" navigation.
- Fixed a few minor bugs.