ENH: Add nice repr for all classes.
Merge pull request #54 from danielballan/restore-repr

ENH: Add nice repr for all classes.
FIX: Frame from Video() should have frame_no.
ENH: Add boolean kwarg 'use_cache' to Video, useful for testing.
TST/CLN: Test that all classes have frame_no attr and that it is correct.
TST: Test that all classes accept a process_func.
FIX/CLN: Common process_func validation for all classes. Add it where missing.

FIX: Untested tifffile code returns Frame, too.
TST: Test two process_func examples on each class. Verify positional arg.