show_circle alpha keyword doesn't work

The `show_circles` docstring says that it's possible to set `alpha` for the circle:
```
    def show_circles(self, xw, yw, radius, layer=False, zorder=None, **kwargs):
        '''
 
        kwargs
            Additional keyword arguments (such as facecolor, edgecolor, alpha,
            or linewidth) are passed to Matplotlib
            :class:`~matplotlib.patches.Circle` class, and can be used to
            control the appearance of the circles.
        '''
```

For me this `alpha` doesn't work at all when I save to PDF and it only works for the edge, not the face when I save to PNG:
```
import numpy as np
from aplpy import FITSFigure
f = FITSFigure(np.random.randn(100, 100))
f.show_circles(50, 50, 20, alpha=0.5, linewidth=30)
f.save('circle_test.pdf')
f.save('circle_test.png')
```

Calling matplotlib myself to plot a circle applies the `alpha` to both the face and edge of the circle for both PNG and PDF:
```
import matplotlib.pyplot as plt
from matplotlib.patches import Circle
circle = Circle((50, 50), 20, alpha=0.5, linewidth=30)
plt.gca().add_artist(circle)
plt.xlim(0, 100)
plt.ylim(0, 100)
plt.savefig('mpl_circle.pdf')
plt.savefig('mpl_circle.png')
```