New Multisource Plugin: Query multiple DBs

New Multisource Plugin:
- Allows SQuirreL to support native multiple source queries where one SQL query spans multiple sources.
- The Plugin supports standard SQL including joins, group by, aggregation, LIMIT, and ordering where tables may come from one or more sources.
- The Plugin will perform function translation where a user requests a function that is not supported on a certain source.
- Added a new UnityJDBC default driver definition that supports this virtualization.

The Plugin has been committed to our SVN repository and will be available in future snapshots and releases.