ENH: add EvokedArray class

Fairly self-explanatory.
@dengemann and I are in agreement regarding <code>create_info</code> in <code>mne.io.meas_info</code>, but we thought it should go in the mne namespace because of the different locations of RawArray, EpochsArray, and EvokedArray.  To me, it would make sense if all were in mne.io, but Denis makes a good case for how it is in the PR now.