system fonts

related to https://github.com/openframeworks/openFrameworks/issues/1832

allows to load fonts from the OS using:

font.loadFont(OF_TTF_SANS,14...)

with constants for:

OF_TTF_SANS, OF_TTF_SERIF and OF_TTF_MONO

it also allows to load fonts by name like:

font.loadFont("Arial",14);