Visualizations

This pull request contains a refactor of `boardApp` to make the process of adding a visualization cleaner. See [readme](https://github.com/Locu/kronos/blob/visualizations/jia/README.md) for detailed information on the structure of a visualization module.

The existing time series and table visualizations have been converted to the new format and a new gauge visualization was added as a simple test. The gauge is a good reference/hello world visualization because it is so basic.