ENH:  DataFrame.value_counts()
Enable value_counts on DataFrame by delegating along axis (ie columns/items) to value_counts as convenience, boiling down to same Series value_counts (Panel is more complicated so not going to attempt for now).

Gets weird with heterogeneous dtypes (b/c union of all unique values) 

@rockg - can you make up some test cases? I already have the implementation. I can put together panel.
