Added new TokenChars field to NGramTokenizers

In 0.90.2 NGramTokenizer and EdgeNGramTokenizer support passing in
character classes in the token_chars field as documented here: http://www.elasticsearch.org/guide/en/elasticsearch/reference/current/analysis-ngram-tokenizer.html

I didn't find any related tests for these classes so there are no tests with this PR.  I did test this out on our live system running 0.90.3.