Add oblique font style

Most backends and fonts understand three font "styles": normal, italic and oblique. Kiva currently only understands the first two.

Adding support for this would require adding
- the appropriate additional constants;
- adding support in the KivaFont trait and the string to font parsers;
- adding support in the Font and FontQuery classes in kiva.fonttools
- adding where backends send the style information directly to the backend (as opposed to using kiva.fonttools to find the actual font file), adding support there

Most of these steps are fairly small and straightforward.