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.
Two things:

* you can store your input colors in a texture using [this generic GLSL snippet](https://github.com/vispy/vispy/blob/master/vispy/visuals/glsl/color.py#L35)
* if you want to avoid textures completely (probably faster), you can have completely standalone GLSL functions that generate the colors on the fly. The template variables `$color_i` are replaced in the `_color` module by the specified input colors and do not go through your shader templating system (the fact that I use the same notation is incidental). These template variables are replaced by hard-coded values for speed.