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/sim/HAL.cpp

@@ -10,6 +10,7 @@
 #include "ErrorsInternal.h"
 #include "HAL/DriverStation.h"
 #include "HAL/Errors.h"
+#include "HAL/Extensions.h"
 #include "HAL/handles/HandlesInternal.h"
 #include "MockData/RoboRioDataInternal.h"
 #include "MockHooksInternal.h"
@@ -203,6 +204,7 @@ HAL_Bool HAL_Initialize(int32_t timeout, int32_t mode) {
   // Second check in case another thread was waiting
   if (initialized) return true;
 
+  if (HAL_LoadExtensions() < 0) return false;
   hal::RestartTiming();
   HAL_InitializeDriverStation();