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.
Ahh, I see below you actually do it this way for colormaps it's possible for, so ignore this question. I'd prefer to have this be `class _Fire`, since users shouldn't need to get to it. Instead they should only look at values from `get_colormaps`. Otherwise there are two ways for users to get colormaps: directly from this namespace, or from `get_colormaps()` -- and worse, some colormaps are in both places and others are only in one.