Changes the default font from Helvetica to Open Sans

This pull request changes the default font from Helvetica to Open Sans. The default font Helvetica seems to be somehow broken (see #14). The Open Sans files are copied out of the latest commit e96682d8be33ab32b0bd4ccfb2ae3a1e167cf666 from the Open Sans FontFaceKit repository https://github.com/FontFaceKit/open-sans .