Chain Chunker and Noun Phrase Mention Finder Addition

Added the ChainChunker and NPChunkMentionFinder
ChainChunker should be modular enough to be used for standard chunking purposes or as a part of the document annotator pipeline for mention retrieval 

Added BILOU and Nested chunking tags and domains to LoadConll2000 and gave it the capability to load files with these encodings that are in the conll2000 format
     Do we want one chunktag whose domain is set instead of three specific classes each with their own specified domain?
Additionally, added BIO to BILOU conversion

Removed faulty gold mention filtering from ConllCorefLoader