Improve color package

* [x] implement `get_color_dict()`
* [x] `LinearGradient` to have a default `x` argument (linearly spaced with `np.arange()`)
* [x] `get_colormaps()` to return the list of color maps
* [x] implement `ColorArray._repr_html_()` to display a color array in the IPython notebook
* [ ] implement `nearest` interpolation in NumPy and GLSL
* [ ] implement more colormaps (see [seaborn](https://github.com/mwaskom/seaborn/blob/master/seaborn/palettes.py), [colorbrewer](http://colorbrewer2.org/), [cubehelix](https://github.com/jradavenport/cubehelix))
* [ ] implement routines to generate special colormaps (diverging, husl, hls...)

![image](https://cloud.githubusercontent.com/assets/1942359/5209124/2bac3536-75bb-11e4-92c3-fbcb0354d810.png)

Close #413. Closes #414.
That's to let users (or us) create their own child classes, like Transforms do.