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
@agramfort we had decided aleady earlier to only allow for dropping channels if preloaded. But there was a bug. We only checked for Raw istanced, not for Epochs. I corrected this, because it made the example fail. I then updated the test, becaus they failed as a consequence of actually putting the droppring API with the preload constraint into practice. Ok?