FIX: Don't import mne in setup.py

This hasn't really caused any problems with MNE-Python, but it was a problem for PySurfer because it required a working X server to install the package. Also, some Linux distributions (Gentoo not sure it it is the only one..) differentiate between dependencies and runtime dependencies (runtime dependencies are only needed when actually using the package, not when installing it). If we import `mne` in `setup.py`, most runtime dependencies automatically dependencies, which is not optimal.