Timecode

PAL 50:
python
> tc = Timecode("50", '00:59:59:49')
> tc.to_systemtime()

I did not implement setters as I don't think there's a meaningful usage or need to them.
Hopefully this may help others understand better Timecodes running at different rates and the relations to the wall-clock time.

The second commit is to preserve the drop frame flag on elementary operations.
python3
> tc = Timecode(29.97, '00:00:00:00', force_non_drop_frame=True)
> assert tc.drop_frame is False
> tc = tc + 1
> tc
'00:00:00:01' before: '00:00:00;01'
> tc.drop_frame
False before: True


Thanks for cubicibo for contribution.

As usual, version `1.4.0` is also released to [PyPI](https://pypi.org/project/timecode/1.4.0/) and running `pip install timecode` will install it.

python
> tc = Timecode("29.97", '00:59:59:29', force_non_drop_frame=True)
> tc.to_systemtime()

(description from 52)
Added a mean to get the timestamps of a given SMPTE Timecode. Two timebases are considered:
- Video system time: `to_systemtime()`
- Real time: `to_realtime()`

NTSC framerates will return different values as the system time is not aligned to the wall-clock time. All others should return the exact same value.

No functional changes in this version, it only contains documentation update to docstring format and clarification on the accepted framerate values.