PR for master

untested get_info written

untested get_info written

added serializer tests, fixed validate_video_prefix

finished most tests

isolated get_info.

finished tests.

get_info for PR

edits for PR

edits for PR

added time modification test

small edits for PR

added dict doc for get_video_info

tidied

added time modification test

small edits for PR

more edits for PR

validates for positive duration, removed profile_id

removed profile_id

more descriptive variable constants

edits for PR

possibly last edits for PR