WIP crfsuite support
some small refactor and document improvement
fix tostr in previous change
abandon CRF++ template in CRFsuite backend

add a default features set for CRFsuite which demostrate how
to create bigram/trigram features with python rather than template.
fix test failures
implement ngrams as global feature function
split the PR into 2 parts

a later PR will add crfsuite base on the newly python-crfsuite(https://github.com/tpeng/python-crfsuite)
missing file in previous change
Merge pull request #10 from tpeng/crfsuite-backend

add base classifier and global ngrams feature functions