Add support for GPOS kerning information parsing

[fpirsch]

This PR provides basic GPOS table parsing.
It reads Pair Adjustment Positioning Subtables (format 1 and 2).
It does not use script and feature information, nor vertical adjustment information.

[davelab6]

Sweet

[fdb]

Wow awesome! I'll look at this ASAP. Thanks!

[fdb]

I've just tried this out on my font stash and it seems to work! Some of the Windows fonts (Candara, Corbel) have no KERN table but do have GPOS information, and this pull request picks them up nicely.

[fpirsch]

Thanks.
Known limitations :
• Current GPOS support doesn't use coverage tables. Which is bad because some fonts (e.g. Null-free) have kerning info for glyphs that are NOT covered. A good implementation should use kerning info only for covered glyphs. I have a fix for this.
• Full GPOS support should use the scripts and features subtables. Currently if multiple kerning subtables are found, well, you just have to hope that the good one is picked up.
• I don't know enough yet about how many kerning pairs we are dealing with in a "standard" font, or how covered glyphs are distributed, so there is absolutely no optimisation. This could be an issue.