Exposed units_per_em function in rusttype
Created by: fschutt
Closes https://github.com/redox-os/rusttype/issues/79
NOTE: This will not yet compile, https://github.com/redox-os/stb_truetype-rs/pull/14 has to be merged previously + the stb_truetype version has to be updated! If this is merged, please release a new version on crates.io, so I can update my dependencies and depend on it from crates.io.