added support for more than one kinect device.

Currently SimpleCV's Kinect class works only with one kinect sensor. However, the underlying freenect module does not have this limitation.

This pull request enables the usage of more than one Kinect sensor with SimpleCV. Since the current Kinect class is just a wrapper for the freenect module, that already has this functionality, I just exposed it to the SimpleCV code.

I added a 'deviceNumber' class variable, that defaults to 0 and can be set in the __init__() method. This variable is afterwards used in the getImage(), getDepth() and getDepthMatrix() methods, in their underlying calls to the freenect functions.

I added some documentation as well.

Example:
```python
# using SimpleCV with two kinect sensors
k0 = Kinect() # use the first kinect
k1 = Kinect(1) # use the second kinect
depth0 = k0.getDepth() # grab the depth read by the first kinect
depth1 = k1.getDepth() # grab the depth read by the second kinect
```