Add font-weight numerical range and fix installed_fonts

Hi mogest,

- Add font-weight [100..900] range support.  It's not compatible with every fonts, but it's better then the actual support.  To get a better one we would need to read the 'OS/2' table from .ttf file to match the css weight with the actual font data.

- Fix the bug where setting weight + style was failing (Only italic was applied).

- Improve the installed fonts algo, now it is better at guessing  subfamily.   (It use field 16 and 17 from table 'name' of .tff file when available).  Tested with Open Sans.

Tell me what you think.  Definitely not ready to merge.  I'm working on it, if you have suggestion.
;-)