Put all extra classpath elements (e.g., plugins) at the end.
Review Request #2210 - Created May 12, 2015 and submitted
See comment in the diff for why. This caused a real problem - zinc was registering a dependency on a plugin jar in the analysis file, which pants then saw as a missing dependency, even though the same jar (at a different path) was in fact a dependency.
Tested that this resolves our missing dependency problem in foursquare's repo.
CI passes: https://travis-ci.org/pantsbuild/pants/builds/62298817