Upgrade to Lucene 4.1

* Removed CustmoMemoryIndex in favor of MemoryIndex which as of 4.1 supports adding the same field twice
* Replaced duplicated logic in X[*]FSDirectory for rate limiting with a RateLimitedFSDirectory wrapper
* Remove hacks to find out merge context in rate limiting in favor of IOContext
* replaced Scorer#freq() return type (from float to int)
* Upgraded FVHighlighter to new 'centered' highlighting
* Fixed RobinEngine to use seperate setCommitData