Rudimentary jar watching

New options watchJars takes a colon separated list of jars
(just the jar names, not paths).  For example:

-Dspringloaded=watchJars=foo.jar:bar.jar

Committed some very basic tests that are passing, they are in
the SpringLoadedTestsInSeparateJVM test class, see the
tests with 'jar' in the name.

testdata/jars/two/Foo.java

@@ -0,0 +1,8 @@
+public class Foo {
+  public static void run() {
+    n();
+  }
+  public static void n() {
+    System.out.println("n() running");
+  }
+}