use compass executable instead of compile_stylesheet

continuing #504, I removed our own `compile_stylesheet` and wrote some compass configuration instead. To clarify: afaiu compass has two parts: sass libraries *and* a better executable/frontend. We already use some of the libraries to generate our css. With this we would make use of the executable, too.

For some reason compass needs relative paths so I could not reuse `adhocracy_code:diazo_theme_rules_dir` but had to create a new buildout variable instead. I hope this can be automized.

Compass allows for specifying an environment ("development"/"production") and applies some default options according to that (e.g. compressing the output). I would like to use that feature but could not yet think of a way to do it. We could use `adhocracy:debug` for that but how do I use that to change variables elsewhere? Is there something like `if` in buildout?

If you could give me some hints about these two issues that would be great.

Before I forget: The point of all this is to have a nice interface to compile css which can handle multiple files/a directory. As discussed in #504 this is not possible with our `compile_stylesheet` script.