Added mpl_transform property to BBox

Demo:

```
from mpl_shapes.bbox import BBox

b = BBox(center=(4, 5), width=2, height=1.5, theta=30.)

import matplotlib.pyplot as plt
from matplotlib.patches import Rectangle, Circle

fig = plt.figure()
ax = fig.add_subplot(1,1,1)

# Draw bounding box itself
r = Rectangle((0,0),width=1,height=1,
              transform=b.mpl_transform + ax.transData,
              edgecolor='0.8', facecolor='none')
ax.add_patch(r)

# Draw a 'circle' inside the bounding box (actually an ellipse)
c = Circle((0.5,0.5), radius=0.5,
           transform=b.mpl_transform + ax.transData,
           edgecolor='red', facecolor='none')
ax.add_patch(c)

print(b.mpl_transform)

ax.set_xlim(0., 10.)
ax.set_ylim(0., 10.)
           
fig.savefig('ellipse.png')
```

gives:

![ellipse](https://cloud.githubusercontent.com/assets/314716/3866686/34d4a3aa-1fdf-11e4-8df1-77c7359bb7ff.png)

cc @ChrisBeaumont