Download guava-bootstrap-jdk5 JAR file with all dependencies

ExecutorService's type parameters changed between JDK5 and JDK6 in a way that makes it impossible for our invokeAll/invokeAny methods to match both at compile time. This project builds a JDK6-like copy of ExecutorService (but with JDK5 compiler settings to ensure that it will work with JRE5 at runtime). It also builds a version of AbstractExecutorService that is equivalent to a JDK5 version but using the JDK6 type parameters for the invokeAll/invokeAny methods just as with ExecutorService. This project's is then used in the bootstrap class path of Guava proper.