turned getColor() into a classmethd

There was a bug in SimpleCV.Color.getRandom() where the documentation didn't match the actual code.

Specifically, the example in the docs made it seem like this method was a classmethod, when in reality the user first had to create an empty SimpleCV.Color instance and then call the getRandom() method on the instance.

I just turned getRandom() into a proper classmethod. I didn't have to alter the documentation, it was already in place ;)

With this change, older code will still work unaffected, but newer code can use the classmethod call.

Example usage:
```python
import SimpleCV

# creating a random color
# old behaviour
dummy_color = SimpleCV.Color()
random_color = dummy_color.getRandom()
# new behaviour (old behaviour works as well)
random_color = SimpleCV.Color.getRandom()
```