Added VineUserIE extractor for vine user timeline

Added vine user timeline extractor using unofficial
vine api user profile and timeline api endpoints.
Enable testing on Python 3.4
[slutload] Add new extractor
[slutload] Add new extractor
[nuvid] Add new extractor
[hentaistigma] Add new extractor
[empflix] Add new extractor
[test/helper] Clarify which field failed an assertion
[mixcloud] Shed API dependency (#2904)
Merge remote-tracking branch 'jnormore/vine_user'
[vine:user] Simplify
Merge remote-tracking branch 'frewsxcv/patch-1'
Credit @jnormore for vine:user (#2888)
[fc2] Add new extractor (Fixes #2877)

This commit has been recreated, since there seems to have been a problem with GitHub; the PR doesn't have a branch.
Credit @hojel for fc2 and other extractors (#2877)
Merge remote-tracking branch 'hojel/slutload'
[slutload] Simplify (#2898)
Merge remote-tracking branch 'hojel/nuvid'
[nuvid] Simplify (#2901)
Merge remote-tracking branch 'hojel/hentaistigma'