ENH: Adding DataFrame.boxplot()
Added DataFrame.boxplot() in similar way as DataFrame.plot() and DataFrame.hist().
Some syntactic sugar ;-)

Thinking about adding DataFrameGroupBy.boxplot(), now this call is transfered to DataFrame.boxplot().
But i would prefer the behavior to be different. When groupby is used i`m interested to see in a single plot boxplots for one particular column for all groupby keys and not in one plot to have boxpots for all columns for one groupby key. An input argument can be used to so both ways of boxplotting can be used.
