Added support for Google AppEngine

Hi,

I restructured base coinbase-java project to add support for Google AppEngine. I also edited readme etc.

Changes:
- added new base project coinbase-java-root
- base project coinbase-java moved to coinbase-java-root
- added new project coinbase-java-appengine

New project coinbase-java-appengine depends on coinbase-java. AppEngine project has own implementation for CoinbaseConnection interface. In ReadMe I added instructions how to use Coinbase on Google AppEngine.

If you have any questions, please let me know.

Dean