[subset] added --recalc-unicode-ranges option through TTFont argument

Hi, 

I tried again to add a --recalc-unicode-ranges option to subset.py, this time via a new 'recalcUnicodeRanges' argument to the TTFont constructor (default is False).

With recalcUnicodeRanges=True, the OS/2 'ulUnicodeRange' values will be recalculated upon save/compile according to the Unicode code points specified in the font's cmap tables.

I put the ranges data and the corresponding updateUnicodeRanges function inside the OS/2 table module.

Please, let me know what you think.
Thank you.

Cosimo