Use separate, simple fixture sites for tests

[diminish7]

Right now, the build test runs against examples/simple-site, which includes testing jade, coffeescript and stylus templates all in one test.

First, that makes our tests rely on the example site, so if we change the example site for documentation / quick-start purposes, it may break our tests.

Second, it lumps all building tests into a single build. Does it make sense to have one fixture site to test jade, one to test coffeescript, one to test stylus etc? This doesn't seem like a huge deal right now since those three things cover separate concerns, but if we add a markdown test, and start adding additional asset types, the single simple site will get cluttered and hard to manage.

[f1337]

👍 decouple all the things

[f1337]

See #61