Inner colorbar & Outer colorbar

Create colorbar at the inner- of outer- side of axes. Unlike the original colorbar command, the location of the original axes is not changed.

```python
import numpy as np
import matplotlib.pyplot as plt

arr = np.arange(100).reshape((10,10))
ax = plt.subplot(111)
im = ax.imshow(arr)

from mpl_toolkits.axes_grid1.colorbar import inner_colorbar
cb = inner_colorbar(ax, im, loc=2)

plt.show()
```

See examples/axes_grid/demo_colorbar.py.

![Alt outer_colorbar](http://dl.dropbox.com/u/178748/axesgrid_figures/demo_colorbar_00.png)

![Alt inner_colorbar](http://dl.dropbox.com/u/178748/axesgrid_figures/demo_colorbar_01.png)