ENH: add spatio-temporal clustering example for sensor data

MNE Python has been capable of this for at least three months. Time to expose it :)

It finds 6 clusters using an F-test, which is justified here as we do not have any ties on a first level analysis
(permuiting condition order does not affect hypotheses).
Here's two of them:

![screenshot 2014-11-05 22 16 44](https://cloud.githubusercontent.com/assets/1908618/4926316/6ff0a2ca-6532-11e4-9f46-e584b08de3e0.png)

For conveniences, I've added the neigho(u)r files from fieldtrip, slightly renamed to make a difference.
Thanks for the attendees of our Paris workshop who motivated me to give this example, especially @matthew-tucker


cc @agramfort
@t3on as you see we had decided this already earlier. Dropping channels also removes the data. But this about dropping the current object in memory, not about the data on disk. If you don't do it for Epochs it willl look up things in the .raw which is still present in such a case. This in turn leads to crashes.