Change fancy indexing to array.take()

When selecting indices along a single axis, `array.take` is faster
than fancy indexing. This becomes noticable when dealing with big
arrays, such as preloaded raw datasets.
Merge pull request #1608 from wmvanvliet/faster-pick_drop_channels

Make pick_channels faster on big datasets
make nose assert_true optional import
fix hard dep on nose
make nose assert_true optional import
fix hard dep on nose