detect buildout and add buildout eggs to sys.path

I am trying to add support for buildout based projects to jedi.

So far this seems to work. But I guess it isn't quite ready to merge yet.

A few questions:
* Can (and should) I use jedi.parser to do the parsing instead of using the ast module?
* Is it okay to write a test directly against _detect_buildout  or should tests be written on a higher abstraction level?