Add font-weight numerical range and fix installed_fonts
Fallback if subfamily not available, fix wrong width_of styled text.
Merge remote-tracking branch 'plehoux/font'

Conflicts:
	lib/prawn/svg/element.rb
	lib/prawn/svg/font.rb
	lib/prawn/svg/parser/text.rb
big refactor to the font system to nicely handle font subfamilies