more iterators in parsers

I changed almost all .parse() methods to return iterators. The exceptions are StanfordParser and MaltParser which I don't know how to handle.

I also changed the methods .parses() and .trees() to return iterators.

Finally I merged commit 8c5f9e7 (removing .prob_parse()), which is the reason why there are so many commits referred in this pull request. Sorry for that