Implements plotfunction #47

[behinger]

I hope the plotfcn_kwargs_filter construct is not too arcane, I didnt know how to do it better. Discussion in #47

I'm further not sure whether to call it plotfnc! or plotfnc

Feedback helpful!

Edit: sorry, changes are against the label fix branch :S

• fix labels, fix eeg_topoplot: labels need to be specified twice #39
• added a backward compatability with warning
• ** removed NaturalNeighbours for now until NaturalNeighboursMethod fail #55 is fixed**
• added example of contourf and surface to docs

[behinger]

looks definitely fancy :)