Plot matrix (and more specifically confusion matrix)
Hello,

I don't find any method to plot a matrix (such as confusion matrix) using "pure" Pandas.

Having a more convenient approach than
http://scikit-learn.org/stable/auto_examples/model_selection/plot_confusion_matrix.html
http://matplotlib.org/examples/pylab_examples/matshow.html
will be great.

```
import pandas as pd
y_actu = pd.Series([2, 0, 2, 2, 0, 1, 1, 2, 2, 0, 1, 2])
y_pred = pd.Series([0, 0, 2, 1, 0, 2, 1, 0, 2, 0, 2, 2])

df_confusion = pd.crosstab(y_actu, y_pred, rownames=['Actual'], colnames=['Predicted'])
```

Something like

```
df_confusion.plot(kind='matrix')
```

It should display matrix with colormap (and a legend of this colormap)

A normalized matrix plot will be great too.

```
def plot_confusion_matrix(df_confusion, title='Confusion matrix', cmap=plt.cm.gray_r):
    plt.matshow(df_confusion, cmap=cmap) # imshow
    #plt.title(title)
    plt.colorbar()
    tick_marks = np.arange(len(df_confusion.columns))
    plt.xticks(tick_marks, df_confusion.columns, rotation=45)
    plt.yticks(tick_marks, df_confusion.index)
    #plt.tight_layout()
    plt.ylabel(df_confusion.index.name)
    plt.xlabel(df_confusion.columns.name)

plot_confusion_matrix(df_confusion)
```

A Pandas class to manage confusion matrix (and also binary confusion matrix) will be great.

Kind regards
