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, right, forgot users might want to implement their own subclasses. Then I can live with using `map` (don't bother with the underscore), but maybe stress in the docstring that `__getitem__` should be used for user-level access, and `map` is there for more low-level computations. I agree we can do `__setitem__` in a subsequent PR if we want it. So I think I'm back to this being ready for merge from my end once you update the `map` docstrings.