Add choose_cubehelix function

Added a simple function that uses the IPython interactive machinery to launch an app that lets you play with the input parameters to the cubehelix system, which can then be used with the `cubehelix_palette` function:

![screenshot 2014-08-23 16 04 56](https://cloud.githubusercontent.com/assets/315810/4022124/05ecc400-2b1a-11e4-81bd-18048ab1e247.png)