Mod stats chart fixes

[HebaruSan]

Problems

• The first month of the year in the mod stats charts is spelled "Janurary":
  

  SpaceDock/frontend/coffee/statworker.coffee

  Line 2 in fa39260

  months = ['Janurary', 'February', 'March', 'April', 'May', 'June', 'July', 'August', 'September', 'October', 'November', 'December']

• https://spacedock.info/mod/175/Interstellar%20Fuel%20Switch#stats has so many versions that the top legend is huge and the bar chart's bars overlaps and its X axis is unreadable:
  
• https://spacedock.info/mod/2524/HumanStuff#stats has a ridiculously long version name that corrupts the bar chart:
  

Changes

• Full month names are replaced with 3-letter abbreviations (e.g., "Jan")
• The old static copy of chart.js is removed, and to replace it, we install the latest version of it with npm
• The old version legend is removed and replaced by the chart's built-in legend, with a small font and small boxes to fit lots of versions
• The new version of chart.js doesn't overlap the bars or X axis labels
• The new version of chart.js handles very long X axis labels better, so the chart is still visible
• The new version of chart.js also allows us to render the bars in different colros, so we match them to the top graph's versions

This screenshot shows a mod with 4 versions, temporarily and artificially dupicated for testing:

Note that this PR includes and is branched off of #310, in order to benefit from that PR's implementation of browserify to install dependencies via npm. The idea is that we would merge that PR and then look at this one. Focus on the second commit for the changes described here.

[DasSkelett]

Yup, looks fine now. I like it!