Added DCO and CONTRIBUTING.md

The DCO is for #290.  Also see [signed-off-by](https://github.com/wking/signed-off-by).

Once in master, CONTRIBUTING.md should show up in pull requests as described [here](https://github.com/blog/1184-contributing-guidelines).

When this gets into master, I'll email the dev list and update the [Contributor's Guide](https://github.com/AnalyticalGraphicsInc/cesium/wiki/Contributor%27s-Guide) to link to it.  In the meantime, I'm going to cleanup the Contributor's Guide.