add k-means plot function

Adds basic functionality for color-coded k-means plots.

Also makes the following changes
* Adds a `show` keyword-argument to the PCA and KMEANS `plot()` function to choose between showing the visualization or returning an html string
* Updates the kmeans `predict()` method to support both RDD or python array input
* Moves the functionality that hid uninformative graph axes from `LinkedView` plugin to new `HiddenAxes` plugin



![screen shot 2014-07-13 at 10 53 57 pm](https://cloud.githubusercontent.com/assets/1074773/3566769/48d59ee6-0b02-11e4-8f68-1a47deb0e86e.png)

I really like letting analyses specify custom colorizers. I think I'd like to refactor this so that we don't duplicate the map logic already in Colorizer.calc, which should be easy by folding the prediction step into the custom get method here.