Update Ant Build logic

The current build.xml file is dated and needs to be updated.

The current build file previously supported building DrJava apps for various platforms (Mac OS X, Windows, Linux) using various versions of Java (5, 6. 7, 8). We now only need to support a single version of DrJava: a generic jar for Java 8 created using Java 8. As a result, massive simplification of the build file should be possible