Add initial typescript definition file for google.visualization.

This is an initial d.ts for the google.visualization API, and is by no
means complete. It contains classes, interfaces, and functions for:

1. Loading visualization packages, and handling package load callbacks.
2. DataTable construction, manipulation, and creation from arrays.
3. Adding event listeners (minimal API, only 1 listener function def).
4. GeoChart construction & drawing.

A small tests file has also been added.