Implement font loading for TrueTypeFonts

This loads TrueTypeFonts using the FreeType library and adds the necessary bits to handle the loaded fonts.

See the new example under `examples/font.js` for how to use this. It's a little bit ugly that the developer has to specify the weight and style of the font manually, but the FreeType library doesn't give something out of the box. It's "just" about parsing the OS/2 table of the font files to extract this information, but I don't wanted to do this with this pull request.

Should fix #53.