Improve color package and add colormaps

* [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
* [x] implement `nearest` interpolation in NumPy and GLSL
* [x] refactor in different files
* [x] add an example notebook with many colormap examples

![image](https://cloud.githubusercontent.com/assets/1942359/5209124/2bac3536-75bb-11e4-92c3-fbcb0354d810.png)

Close #413. Closes #414. Closes #646. To be continued in #648.
This docstring should follow our standard / numpy / scipy one-line top description, with additional description on a subsequent line. It also needs `Parameters / Returns` fields. Same with `LinearColormap` above.

Now that I think more about it, we might as well combine these two classes. They're both interpolation schemes found in `scipy.interpolate,interp1d', where one is equivalent to `kind='linear'` and the other `kind='zero'`, which is the most likely function other people will be familiar with for interpolation:

http://docs.scipy.org/doc/scipy-0.14.0/reference/generated/scipy.interpolate.interp1d.html

This would also make it easier to add other interpolation schemes in the future if we want, or even change modes of the colormap on the fly, which would be neat.