Test for "amg" mode for spectral clustering added.
docfix: spectral_cluster doesn't return n_centers
pep8
Spectral will raise an error if the mode is set to amg and pyamg is not available
Test that an unknown mode raises the appropritate error
Merge remote-tracking branch 'origin/master'

Conflicts:
	sklearn/cluster/spectral.py
	sklearn/cluster/tests/test_spectral.py
Merge remote-tracking branch 'upstream/master'
Merge remote-tracking branch 'upstream/master'
Merge branch 'master' of github.com:robertlayton/scikit-learn

Conflicts:
	sklearn/cluster/tests/test_spectral.py