register Axes3D along with other projections

The docs state the following for `mplot3d` as preferred usage:
```python
import matplotlib.pyplot as plt
from mpl_toolkits.mplot3d import Axes3D
fig = plt.figure()
ax = fig.add_subplot(111, projection='3d')
```
Because Axes3D is only registered as a projection in `mpl_toolkits.mplot3d.axes3d` one has to import from there even though the module is otherwise integrated rather seamlessly. It feels a bit odd to have an unused import as the preferred usage.

How about registering the projection earlier, within matplotlib, thus obviating this import? As far as I can see mplot3d is always there when matplotlib is installed (I put an `except ImportError` anyway to make double sure to not break anything). Well.. just cosmetics, I guess, so feel free to just close.

If this gets a thumbs up, I'll also change the docs accordingly.