WIP: add Forward class with repr

To be able to see important information regarding the gain matrix, i.e.:

    In[1]: fwd
    Out[1]: <Forward | MEG channels: 118 | EEG channels: 63 | Source space: Surface with 8171 vertices>

I largely copied the structure from the <code>Info</code> class in <code>mne.io.meas_info.py</code>, which @dengemann tells me he wrote.  Does iteration have utility there but not here?  If so, sure.