Topology Expansion + Loading Indicators

This pull request is a complete overhaul of the previous code to allow for dynamic topology expansion, and also adds a loading indicator feature.

The majority of Javascript code has been externalized into three different modules under the `versastack` namespace. 

`versastack.model.js` reads a graph in raw JSON format and does some processing to parse it into usable data structures and models by adding relationships between nodes.
`versastack.loading.js` keeps track of items that are asynchronously loading or processing like the model module and displays a loading indicator image in the meanwhile. For now there is not much loading delay, but it may be useful in the future.
`versastack.adminview.js` polls the loading module until all the data is processed, then initializes a force graph to display the data and keeps track of which parts of the graph are displayed by handling events.

A `sources.txt` file under the resources/ folder lists all the image sources.