Upgrade to Lucene 3.6.1

It's a straight-forward process, we just need to update the poms and the code in certain places where it establishes compatibility with lucene version 3.4 (to be changed to 3.6).

Besides getting the latest improvements, the upcoming Solr search backend will be compatible with this latest 3.x (3.6.1) Lucene branch. To use Sorl 4.0 we need Lucene 4.0 but that upgrade requires some heavier refactoring due to the API changes in Lucene 4.0.