Minor optimization
[subset] Implement format14 variation selectors
[subset] Fix PairPos subsetting

Ouch!

Fixes https://github.com/behdad/fonttools/pull/118
[subset] Unbreak after e898881ed12d7ce96948f2905d725847c76bc9e8
Speed up cmap format 4 loading

From quadratic to linear.  Sigh.
Micro-optimize cmap subtable format 4 loading
Further micro-optimize cmap subtable format 4 loading
Use .extend() syntax instead of += for sets
Micro-optimize hmtx/vmtx loading
Add a few __delitem__ implementations