Improved structure to Bokeh plot generation code

[wafels]

The code hv_stats.py creates interactive plots that allow users to visualize various parts of Helioviewer service usage. The need for this code was driven partly to understand how users use Helioviewer at GSFC and partly to generate plots for a paper. The file contains code that generates many different plots. The code should be re-structured to be more modular, which will make it easier to maintain and develop in the future. Further, the GSFC specific parts should be extracted so that the code is agnostic to the helioviewer server location.

[dgarciabriseno]

I expect this will be tackled if/when we need to update the plot generation.