New maven project "stratosphere-addons".

This pull request introduces the new sub-project structure suggested in https://github.com/stratosphere/stratosphere/issues/199#issuecomment-29057242

This PR moves pact-array-datamodel and pact-hbase to stratosphere-addons


I will also add the avro input format and the simple hbase output format soon.
Pull Request https://github.com/stratosphere/stratosphere/pull/279 depends on this, because I would like to add the jdbc input format as an addon.