cleaning up some of the documentation of at-root with a slightly more complex example. sass/sass#1462
Merge pull request #1463 from hcatlin/at-root-docs

At-Root Documentation Fix
Create appveyor.yml to run Windows CI service

Add Appveyor configuration file so this can be built on the Appveyor CI service (Windows 2012).

The build matrix is emulated by specifying two platforms to build on; note that these are both still the stock x86-ruby running on the x64 windows version.
larger build matrix on AppVeyor

new versions of Ruby (with DevKit) have been installed on AppVeyor VM's:
  - 1.9.3 x86 (C:\Ruby193 - default in PATH)
  - 2.0.0 x86 (C:\Ruby200)
  - 2.0.0 x64 (C:\Ruby200-x64)

this updates to run tests on those.

Note: the 2.0.0 versions don't have bundler installed by default, so we need to do that now; this may change in the future.