ENH: Updating a dataframe counting the cells that will be updated.
Counting the cells that were updated when an update is performed to a dataframe.

Currently there are no commands to count the number of cells being updated so you can put it in a log or something when `df.update(df2)` is performed.

See:
http://stackoverflow.com/questions/23102757/python-pandas-updating-dataframe-and-counting-the-number-of-cells-updated
