use webjars-locator

and eliminate webjars.sbt for now. some form of it might come back
later as part of support for V8-based testing and/or browser-based
testing

for gory details, see discussion on HipChat today
Merge pull request #83 from NetLogo/webjars-locator

replace webjars.sbt with webjars-locator
Recategorizing failing language test
Reordering many of the freebie language tests
Creating a handy, dandy 'jsengine' package
Introducing Benchmarker 1.0
Adding version number of JS engine to benchmark

It just makes sense to include version numbers of external libraries
into the benchmarks, since changing version numbers can change speed.
Minor package-pushing (refactoring)
Pumping up the private sector
Adding SpiderMonkey support