WIP: rewrite Measure to be simpler, faster, and generic
Get rid of the silly and seldom used compose and combine operators.
More work converting to the new Measure system
SVG backend more or less working again.
More work. Basic plotting works now.
Fix cairo backend issues.
Merge branch 'master' into compose2
Remove Mustach import, it's no longer used.
Make d3 work with type generic measures.
More changes to support generic types in BoundingBox, etc.
Overhaul of a lot of code handling units, measures, and such.

This is partically as an overally cleanup, but primarily to add support for
specifying coordinate systems in terms of types other than Float64. This makes
in easier to extend Gadfly to plot other types, without converting overything to
floats.

I've also removed the silly '|' and '<<' operators.