CAS-1205: Add a custom_messages language bundle, by default non existing and empty

in order to allow adopters to keep their own custom messages without having to deal
with cas bundles in order to ease the process of upgrades.

This issue is closely related to CAS-1347

https://issues.jasig.org/browse/CAS-1205
CAS-1205: configured loading of the basenames to use custom first
CAs-1205: Removed unneeded condition, and fixed references to the custom bundle in tests
Merge pull request #333 from Unicon/CAS-1205

CAS-1205: Add a custom_messages language bundle