Limited Composite PK support

[comtel2000]

See:
https://groups.google.com/forum/m/#!topic/activejdbc-group/mSgxD1zwJIs

[comtel2000]

Please skip the first jdk6 commit. I got strange issues in my eclipse IDE with the default jdk5 setup.

[ipolevoy]

@comtel2000 , I added and cleaned your code: #405.
Also, added docs here: http://javalite.io/surrogate_primary_keys#what-about-composite-pks
it is part of the framework now, thanks for your contribution!

[swolf91]

@ipolevoy Did you plan to add relationship support in the near future? At least it would be great to have a working include method, so that entries can be cached.
Is there more work to do as changing the orderBy properties?

[ipolevoy]

@boeserwolf91 , currently no plans to add relationships for composite keys. The ActiveRecord too does not support composite keys out of the box (available as a separate gem outside Rails project). If we get enough requests for this, we might consider.