Bring back the gazebo plugins (#1063)

The models and meshes are not included. We will need
to find an alternate way to reintegrate these and use them.

* Add simulation/gz_msgs back, and build with Gradle.

* Add back in the frc simulation plugins for gazebo.

* Add a new shared library, halsim_gazebo.

This library will become the interface between the
HAL sim layer and gazebo.

* Preserve the first channel number used in created Encoders in the Sim MockData.

This allows us to use the DIO channel number to connect with simulated encoders.

* Have the HAL Simulator set the reverse direction on creation.

This enables a simulator to be aware of the direction.

* Add a drive_motor plugin.

This is a bit of a 'magic' motor, which allows us to build robot
models that drive in a more realistic fashion.  It does this
by apply forces directly to the chassis, rather than relying on
the complex motion dynamics of a driven wheel.

This in turn allows the model to reduce wheel friction,
reducing scrub, and allowing for a more natural driving experience.

simulation/frc_gazebo_plugins/README.md

@@ -0,0 +1,45 @@
+Gazebo plugins for FRC
+======================
+
+These plugins enable the use of the Gazebo physics simulator to model robots,
+and then use WPILIB programming libraries to control those simulated robots.
+
+At the present time, this only builds on Linux, and has a fairly complex
+installation process.
+
+You must have the gazebo development packages available on Linux.
+The gradle build system will automatically skip building this directory
+if those requirements are not met.
+
+This command:
+
+    `./gradlew build -PmakeSim`
+
+will force it to attempt to build.
+
+TODO:  The meshes for the models are large, and are not hosted in
+the main allwpilib repository.  An alternate method of hosting
+and using is being built at the time of this commit. You must
+manually locate and copy the the models and world from that
+either into /usr/share/frcgazebo or into build/share/frcgazebo
+in order to use this facility.
+
+Once you have built it, then a command like this:
+
+  `simulation/frc_gazebo_plugins/build/bin/frcgazebo PacGoat2014.world`
+
+should run the Gazebo simulation using our plugins against the
+2014 model, with the PacGoat robot on the field.
+
+The halsim_gazebo simulation library will provide a method for
+running robot code to control a simulated robot.
+
+
+TODO:  Package the plugins more nicely, and then put instructions here.
+
+TODO:  Build and package the plugins for Windows.  The key there is to get gazebo
+working on Windows.  This link was a useful trove of information on Windows/Gazebo
+issues as of September, 2017:
+  https://bitbucket.org/osrf/gazebo/issues/2129/visual-studio-2015-compatibility
+
+Note that the sense remains that this will be difficult at best.