Adding skll_convert to scripts and fixing some bugs

1. `skll_convert` can convert between jsonlines, megam and tsv formats. 
2. Fixed bug in _megam_dict_iter where labels for instances with all zero features were being incorrectly set to None.
3. 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.