Fixing comment
Allowing Song models to have a non-integer tempo

To allow swung songs to have a more accurate tempo after they
are transformed.
Allowing swung songs to be converted to a fractional tempo

To avoid them drifting out of time due to an inaccurate
integer tempo.
Removing unnecessary parens
Adding AudioUtils.step_sample_length tests for fractional tempo
Adding SongParser tests for song with fractional tempo