Get current milliseconds from timecode and implement 2:3 pulldown/up

closes #3 and closes #5
* pulldown/up works only when converting between 23.976 and 29.97
* Add toMilliseconds() function and tests
* Optimize getFrame() calculation
* Add check for valid framerate
* Update readme with supported framerates and bump version number
* Fix casting framerate from 23.98 to 23.976