Removed a TODO that didn't actually show big improvement.
Documented an idea for avoiding too much hashing.
Profiled the incremental java compilation and tackled the first bottleneck: the detection of duplicate classes in the classpath was way too slow.
Documented one more thing TODO.
Documented incremental java compilation in the DSL for JavaCompile task.
Documented incremental java compilation in the user guide.
Added release note for incremental java compilation.