Pull request for the Extensions interface only (#655)

* Modify halsim to be able to load extension libraries if they are available.

It will read the list of libraries to try from the HALSIM_EXTENSIONS
environment variable.  Multiple libraries can be given if separated
by ';' (Windows) or ':' (Unix).

The library must have an 'HALSIM_InitExtension' method that returns >= 0 on success.

The library is expected to use the interface expressed by
  hal/src/src/main/native/include/MockData

* Add a simple halsim library that just prints robot values.

This makes a good test bed for cross platform purposes,
and provides the ultimate in light weight simulators.

This initial version only prints PWM values.

hal/src/main/native/include/HAL/Extensions.h

@@ -0,0 +1,25 @@
+/*----------------------------------------------------------------------------*/
+/* Copyright (c) 2017 FIRST. 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.                                                               */
+/*----------------------------------------------------------------------------*/
+
+#pragma once
+
+/**
+ * HAL Simulator Extensions are libraries that provide additional simulator
+ * functionality, such as a Gazebo interface, or a more light weight simulation.
+ *
+ * An extension must expose the HALSIM_InitExtension entry point which is
+ * invoked after the library is loaded.
+ *
+ * The entry point is expected to return < 0 for errors that should stop
+ * the HAL completely, 0 for success, and > 0 for a non fatal error.
+ */
+typedef int halsim_extension_init_func_t(void);
+
+extern "C" {
+int HAL_LoadOneExtension(const char* library);
+int HAL_LoadExtensions(void);
+}