Support for different Hadoop versions (yarn!) using activation by properties

This pull request is a fix for https://github.com/dimalabs/ozone/issues/95
The results of this PR are mostly based on the work and research of [Andre](https://github.com/andrehacker).

We implemented maven profiles similar to other large projects that depend on hadoop (giraph, hbase).
Similarly to hbase, we chose to supply a little script that generates specialized POMs for the  'hadoop2` profile.
The above mentioned issue contains an extensive discussion of the problem. 

The README lists all relevant commands and the travis build system supports the new profile activation