Create dummy wpiutil library. (#84)

This will allow dependencies such as wpilibc to update to use wpiutil
without breaking "normal" ntcore static library use in the meantime.

This commit also restructures the gradle files by creating a new
(placeholder) wpiutil project, and moving the ntcore project into
a separate gradle file.  Added toolchains/native.gradle (refactored from
ntcore).

Also fixes ntcore skipJava on Windows by providing an alternate .def file
for this case.

wpiutil/unittest/main.cpp

@@ -0,0 +1,15 @@
+/*----------------------------------------------------------------------------*/
+/* Copyright (c) FIRST 2015. All Rights Reserved.                             */
+/* Open Source Software - may be modified and shared by FRC teams. The code   */
+/* must be accompanied by the FIRST BSD license file in the root directory of */
+/* the project.                                                               */
+/*----------------------------------------------------------------------------*/
+
+#include "gtest/gtest.h"
+
+int main(int argc, char **argv)
+{
+  ::testing::InitGoogleTest(&argc, argv);
+  int ret = RUN_ALL_TESTS();
+  return ret;
+}