Add base_font_size property to RstDocument

"base_font_size" sets the font size for the biggest title, other font
sizes are derived from that.
Merge branch 'master' of https://github.com/akloster/kivy into akloster-master
rst: rename root to document, and add versionadded token to base_font_size
Merge branch 'master' of ssh://github.com/kivy/kivy