collapsed _iter/iter_ and non-iter methods; merged nbest_parse into parse; changed return of parse from a single item vs None to an iterator
made parse return a list instead of an iterator, cf issue #614
Parser bugfixes
Minor fix for the chartparser_app
Made parsers return iterators instead of lists (reverting #cfd6dd5)
Merge pull request #627 from heatherleaf/iter

Parser bugfixes
Made the backtracking rd-parser use iterators instead of lists
Replaced Chart._choose_children with itertools.product
Made .parses() and .trees() return an iterator
Merge branch 'develop' into iter
Merge pull request #634 from heatherleaf/iter

more iterators in parsers
adopting new parser api proposed by Peter Ljunglöf
Continuing to finalise the parse->read name change
fixed some merge conflicts
align parser methods with updated API
align parser methods with updated API
align parser methods with updated API, incorporate read_cfg into ContextFreeGrammar.read, and same for read_pcfg and read_fcfg