Add zanata-overlay submodule.

This submodule takes care of building an overlay zip file that when extracted on top of the different supported JBoss distributions, will supply the necessary components for Zanata to run. In this iteration it contains:
- Necessary jboss submodules
- Configuration and startup scripts for zanata
- Installation scripts which fetches the war file from the web and configures some parts of the application.

Currently, it builds 2 distributions:
Wildfly 8.1
EAP 6 (6.3 recommended)

This pull request goes hand in hand with https://github.com/zanata/zanata.github.io/pull/13
Caught: java.lang.NullPointerException: Cannot get property 'connection-url' on null object
java.lang.NullPointerException: Cannot get property 'connection-url' on null object
	at InstallZanata.run(InstallZanata.groovy:95)