Scatterplot Method added

This is a simple pull request that creates a shortcut for scatter plots from a df.  This is in reference to  #2215  (and  #1527) and follows the same pattern as df.hist(). 

    #where you would previously have had to do:
    from pandas.tools.plotting import scatter_plot 
    scatter_plot(df, x = "x", y = 'y')
    
    #you can now do
    df.scatter(x="x",y="y")

    #example
    import pandas as pd
    import numpy as np
    df = pd.DataFrame( np.random.randn(100,4))
    df.scatter(x=1,y=2)
    
I followed the same pattern as df.hist() even though it looks like you are intended make a class object for scatterplots that can be passed to the plot_frame() function as is the case for BarPlot and LinePlot. In any case, this works and may be useful.

best,
zach cp