rough first pass at Stat.qq

Just calls qqbuild() on aes.x and aes.y, and writes the .qx and .qy
to aes.x and aes.y.
[WIP] extend Stat.qq to work with Distribution and add tests

Requires modifying some basic definitions (types that are allowed to
be bound to aesthetics, and skipping scale application on the first
pass).

Needs more work on non-default Scale application
scales work for Distributions now, too (with tests)
document Stat.qq
Merge pull request #391 from kleinschmidt/dfk/stat_qq

Add Stat.qq for QQ plots (#375)