[REEF-43]: Get version number from pom

The REEF interface defines a method getVersion(), and a hard-coded
variable REEF_VERSION, set to "0.6-SNAPSHOT".
However, it has been out of date and is not a viable solution indeed.
This PR defines an injectable class Version to get the version number
from pom (via a properties file).

JIRA:
[REEF-43] https://issues.apache.org/jira/browse/REEF-43

Author: chobrian <chobrian@apache.org>

Closes #19