Use Google Charts instead of NVD3.js.

The latter had some performance regressions after updating chrome.
These regressions caused browsers to crash in the worse cases.
Migrating to the Google Charts API provides a similar implementation
with a much nicer API. We get:

- Consistent colors for each weapon
- Clickable data points (events on selection)
- Tons of customization via a simple JSON options object
- No additional source files
- No need to buffer weapon stats JSON data with "zero" entries

Right now the AJAX requests are still using D3.js style. The next
step is to migrate them to jQuery or vanilla JS.
Remove NVD3 files. Goodnight, sweet prince!

You served us for a long time. I hate to see you go! Maybe we'll
meet again some time in the future.