REEF-43: Get version number from pom (via properties file)

This addresses https://issues.apache.org/jira/browse/REEF-43

The injectable class `Version` now gets the version number from pom (via a properties file).

Those are two different versions, right? One is the API version of our REST API. The other is the version of REEF. The former can be short and incremented on a completely different schedule.