ENH: Add raw plotting

Currently it plots raw data that has been preloaded (or not), with scrollbar-like functionality across both channels and time, as well as up/down/left/right arrow key functionality. It can also be used to dynamically show the effects of projectors.

I think some adjustments could be made in terms of layout tightness, but this should be good for initial use. People are welcome to try it -- should be able to do `mne.viz.plot_raw(raw)` or `raw.plot()` to use it.