Add writing capabilities for triangle files to io.Surface

Produces files identical to freesurfer up to the point that PySurfer
reads. There is additional information after a freesurfer generated
file, but PySurfer displays these files fine.
RF Use nibabel write_geometry function
Merge remote-tracking branch 'upstream/master'

Conflicts:
	surfer/io.py
RF Use nibabel implementations of IO functions where available
RF Use nibabel.freesurfer.read_ functions in test_viz and examples
BF nibabel.freesurfer.read_label is not equivalent; revert
STY Line split unnecessary
Merge pull request #47 from effigies/master

RF Use nibabel IO functions where available