Better repr for Bboxes.

`Bbox`es have fairly ugly reprs right now, this small patch tries to improve that.

Before:
```
$ python -c 'from matplotlib.transforms import Bbox; print(Bbox([[0, 0], [1, 1]]))'
Bbox('array([[ 0.,  0.],\n       [ 1.,  1.]])')
```

After:
```
$ python -c 'from matplotlib.transforms import Bbox; print(Bbox([[0, 0], [1, 1]]))'
Bbox(x0=0.0, x1=1.0, y0=0.0, y1=1.0)
```