added ZH for chinese ngrams documentation
fix Trie.keysWithPrefix throwing on not found
fix Trie.keysWithPrefix throwing on not found
Merge pull request #149 from moos/master

fix Trie's keysWithPrefix() throws if string not found
Merge branch 'master' of https://github.com/liwenzhu/natural into liwenzhu-master
added unit test for #150
Merge branch 'master' of github.com:NaturalNode/natural
Merge pull request #3 from NaturalNode/master

sync origin repo