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/src/clock/headers/clock.h

@@ -0,0 +1,58 @@
+/*----------------------------------------------------------------------------*/
+/* Copyright (c) 2016-2018 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
+
+#include <string>
+
+#include <gazebo/gazebo.hh>
+#include <gazebo/physics/physics.hh>
+#include <gazebo/transport/transport.hh>
+
+#include "simulation/gz_msgs/msgs.h"
+
+/**
+ * \brief Plugin for publishing the simulation time.
+ *
+ * This plugin publishes the simualtaion time in seconds every physics
+ * update.
+ *
+ * To add a clock to your robot, add the following XML to your robot
+ * model:
+ *
+ *     <plugin name="my_clock" filename="libclock.so">
+ *       <topic>~/my/topic</topic>
+ *     </plugin>
+ *
+ * - `topic`: Optional. Message will be published as a gazebo.msgs.Float64.
+ *
+ * \todo Make WorldPlugin?
+ */
+class Clock : public gazebo::ModelPlugin {
+ public:
+  /// \brief Load the clock and configures it according to the sdf.
+  void Load(gazebo::physics::ModelPtr model, sdf::ElementPtr sdf);
+
+  /// \brief Sends out time each timestep.
+  void Update(const gazebo::common::UpdateInfo& info);
+
+ private:
+  /// \brief Publish the time on this topic.
+  std::string topic;
+
+  /// \brief The model to which this is attached.
+  gazebo::physics::ModelPtr model;
+
+  /// \brief Pointer to the world update function.
+  gazebo::event::ConnectionPtr updateConn;
+
+  /// \brief The node on which we're advertising.
+  gazebo::transport::NodePtr node;
+
+  /// \brief Publisher handle.
+  gazebo::transport::PublisherPtr pub;
+};