Add abstract table class

Efficient parsing of the kern table

Kerning by classes
This implementation uses the PairPosFormat1 to store kerning values in
TTF files and has a new kerning display that allows the user to define
kerning classes.

Some code to do PairPosFormat2 is included as well but it seems like
the PairPosFormat1 is the better solution.

Merge branch 'master' of gitorious.org:birdfont/birdfont

mac issues, including a more permissive license for libbirdfont

Subdirectory for open font format tools