Use different interface for setting LUT

[mwaskom]

Closes #188

There is an inconsistency where the previous method wanted values in [0, 255] while the current method wants values in [0, 1]. I decided to handle this within each method to otherwise maintain prior API.

I generated the docs on my Ubuntu system and everything looks good. It would be best to have someone else on a different system verify that the example gallery does not change with this PR.

[coveralls]

Coverage increased (+0.05%) to 78.413% when pulling 8515afa051da9532a858129ebcc23ab08a727879 on mwaskom:fix_lut_update into 69e969b3698314470ce37f608651dd7493316db9 on nipy:master.

[agramfort]

fine with me

[larsoner]

Thanks @mwaskom