Fix bug affecting video transcripts in XMLModulesStore courses
Add sjson and srt to type map.
Merge pull request #3648 from edx/oleg/fix-download-transcript

Add mime-type for srt and sjson when importing course.
Merge pull request #3725 from carsongee/cg/transcript_xmlmodulestore_fix

Fix bug affecting video transcripts in XMLModulesStore courses
Tolerance expressed in percentage now computes correctly. [BLD-522]
Modified and extended tests on percentage expressed tolerance.
Addressed comments.
All tests pass now.
Added tests for compare_with_tolerance
Put unit tests of util in their own file.