MicroDVD implementation and tests, some bugfixes

- MicroDVD parser and writer
- SRTFormat -> SubripFormat
- better heuristics for Subrip
- as documented, set SSAFile.original_format in parse() methods
- add ms_to_frames() function (w/ tests)
- fix fps-related bug in SSAFile.save()
- fix broken can it read? can it write? detection in register_format()