Added legend.framealpha to rcParams, as mentioned in axes.legend doctoring

I like having translucent legend frames. In _axes.py, I saw:
framealpha : None or float
            Control the alpha transparency of the legend's frame.
            Default is ``None`` which will take the value from the
            ``legend.framealpha`` :data:`rcParam<matplotlib.rcParams>`.

So, I added lines in legend.py and rcsetup.py to implement this capability.