All solutions

This is still experimental work. It attempts to fix the fundamental problem of https://github.com/conda/conda/issues/491, which is that conda only looks for package solutions for the latest version of each package specified. Hence, if there is a possible solution but it doesn't contain the newest versions of everything, you will get "unsatisfiable package specifications". 

This pull request changes it so that all versions are checked. This results in many more solutions, but they are then sorted by version and minimality, which typically results in only one solution again (unless there really are multiple possibilities).

Right now, there are issues with things like numpy-1.7.1-py27_p0 vs numpy-1.7.1-py27_0, which are both comparing as the same package (the former is the mkl version of numpy). I'm still trying to figure out the best way to fix this.