ENH: add EvokedArray class

Fairly self-explanatory.
Sure, but shouldn't the reference file be changed also?

Currently:

    :py:mod:`mne.io.array`:

    .. autosummary::
       :toctree: generated/
       :template: class.rst

    RawArray

    Functions:

    .. currentmodule:: mne.io.array

    :py:mod:`mne.io.array`:

    .. autosummary::
      :toctree: generated/
      :template: function.rst

    create_info

Wouldn't we need to change the instances of <code>mne.io.array</code> to <code>mne.io</code>?  On the same issue, would it make sense to move <code>create_info</code> to the mne namespace?