Codepoints

This adds codepoints support to node-fontnik. A sync function called
`codepoints(str:facename)` returns an array of numbers indicating
Unicode code points covered by the font. Implements #56
Yes, but...

Ideally this function would take one font as an argument - I don't know how to do that with `font_manager`, so hence this. Getting coverage for a fontstack could be trivally done in JavaScript.