ENH: add EvokedArray class

Fairly self-explanatory.
So ... <code>create_info</code> in the mne module (where it is now) and <code>RawArray</code> (<code>EpochsArray</code>) (<code>Evoked_Array</code>) in <code>mne.io</code> (<code>mne.epochs</code>) (<code>mne.evoked</code>)?