Remove `prepare_display` from docs
Add more documentation

- Geom.contour
- Guide.colorkey
- Guide.title
- Guide.xlabel
- Guide.ylabel
- Guide.xticks
- Guide.yticks
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 branch 'add_docs' of https://github.com/darwindarak/Gadfly.jl into darwindarak-add_docs
Website things.
Merge pull request #391 from kleinschmidt/dfk/stat_qq

Add Stat.qq for QQ plots (#375)
Remove references to `@plot`.
Fix dodged horizontal histograms.
Ambiguity warnings. :rage1: Fixes #378
Make Stat.histogram compatible wih Geom.line. Fixes #390
Fix potential type error in tick statistic.
Tests. Fix subplot_grid with both axis free.
Tests. Fix a bug in histograms over categorical data.
Forbid x y of unequal length in Geom.bar. Fixes #338
Fix for non-number types in Geom.bar. Fixes #355.
Require Compose 0.3.6