Support for non-24 fps timecode

[jminor]

Opentime should support all of the standard video editing frame rates: 24, 25, 30, 48, 60 and the drop frame variants 23.98, etc.

[ssteinbach]

@bashesenaxis reported a version of this with #55 -- pinging him here so he can see this issue.

[reinecke]

I think I have some pretty good code for this in C I could port to opentime. Does anyone have good reference data to use for unit testing (i.e. a list of times or frame numbers and the timecode they should read)?

[ssteinbach]

@bashesenaxis may have some, I think he hit this while working on another bug

[bashesenaxis]

The bug that I hit was in:
https://github.com/PixarAnimationStudios/OpenTimelineIO/blob/master/opentimelineio/opentime.py#L465
where 24.0 is hard-coded. I believe that should be rate instead?
I encountered this when writing unittests for the xml adapter. The data I used was the supplied premiere_example.xml, which is at 30 fps. I haven`t done any tests with drop frame data.

[jminor]

@bashesenaxis can you take a look at @reinecke's fix reinecke@4bc9233 to see if this handles the frame rates you need?

[bashesenaxis]

Yeah, that looks good to me.
Following link is the only place I have been able to find a hard-coded rate, and this pull request fixes that.
https://github.com/PixarAnimationStudios/OpenTimelineIO/blob/master/opentimelineio/opentime.py#L493