ENH: Improve epochs drop_log

@agramfort / @Eric89GXL here I outlined the idea exposed in #592.
It will add a new logging message to the dorp log indicating that the particular epoch was dropped during count equalization. 

For reconstructing the full index of dropped epochs one can then say: 
`[i for i, e in epochs.drop_log if e]` 

Which can be very useful when e.g. integrating reaction time data into the analysis.

We might also consider adding a drop_index attribute but I'm somewhat reluctant of adding another attribute ...

wdyt?

D