Added write_feature_file function to skll.data, which makes it simpler to write .jsonlines, .megam, and .tsv files. Also added unit tests for megam and tsv file merging.
Fixed Python 2/3 compatibility annoyances with DictWriter.
Fixed issue where unit tests weren't creating directories.
Fixed issue where unit tests weren't creating directories.
Merge branch 'feature/write_feature_file' into develop