Improve color package

Not started yet, creating this as a placeholder.

* [ ] `get_color_dict()`
* [ ] `LinearGradient` to have a default `x` argument (linearly spaced with `np.arange()`)
* [ ] allow `LinearGradient[array]`
* [ ] improve performance of `__getitem__` (by avoiding the loop if possible?)
* [ ] replace `__getitem__` by `__call__`? More intuitive maybe.
* [ ] move the color maps from `_color.py` to the GLSL repository
* [ ] `get_colormaps()` to return the list of color maps
* [ ] implement `ColorArray._repr_html_()` to display a color array in the IPython notebook

ping @Eric89GXL