Added implementation of AhoCorasick for lexicon lookups.
Added a find exact match method, and updated the default lexicon methods
to use it.

This replicates the current PhraseLexicon functionality exactly, by
treating the AhoCorasick automaton as a Trie.
Change the lexicons to use TriePhraseLexicon instead of PhraseLexicon.
Changed the NER classes to correctly use the new lexicons.
Logging bugfixes.
Bug fixes to the mention finding and tagging code.
Merge pull request #189 from Craigacp/master

Aho-Corasick Trie based Lexicons