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)

Think it's worth factoring this into a time series method inside thunder.viz.plots? Like for scatter, etc.