Added the first version of a universal feature converter script called
`skll_convert`.
Merge branch 'develop' of https://github.com/EducationalTestingService/skll into feature/add_skll_converter
Added unit test for skll_convert script. Fixed bug in _megam_dict_iter
where labels for instances with all zero features were being incorrectly
set to None. Also fixed bug in _tsv_dict_iter where features with zero
values were being retained with values set as '0' instead of being
removed completely.
Potential fix for Python 2.7 str/unicode nastiness
Merge branch 'develop' of https://github.com/EducationalTestingService/skll into feature/add_skll_converter
Fixed issues due to python 2.7 DictReader not being unicode friendly.
Removed stray pdb trace.
Merge pull request #46 from EducationalTestingService/feature/add_skll_converter

-  `skll_convert` can convert between jsonlines, megam and tsv formats.
-  Fixed bug in `_megam_dict_iter` where labels for instances with all zero features were being incorrectly set to `None`.
-  Also fixed bug in `_tsv_dict_iter` where features with zero values were being retained with values set as '0' instead of being removed completely. This caused `DictVectorizer` to create extra features, so results may change a little bit if you were using .tsv files.