Add Robot Spawn Pose Launch Args

Adds x,y,z,yaw arguments to set pose where the robot spawns.
New arguments are defaulted to their previous values.

Example:
```
roslaunch ridgeback_gazebo ridgeback_world.launch x:=10 y:=10
world_name:=/path/to/my/world.sdf
```

ridgeback_gazebo/launch/ridgeback_world.launch

@@ -4,6 +4,12 @@
   <arg name="headless" default="false" />
   <arg name="world_name" default="$(find ridgeback_gazebo)/worlds/ridgeback_race.world" />
 
+  <!-- Robot Spawn Pose -->
+  <arg name="x" default="0"/>
+  <arg name="y" default="0"/>
+  <arg name="z" default="0.1"/>
+  <arg name="yaw" default="0"/>
+
   <!-- Configuration of Ridgeback which you would like to simulate.
        See ridgeback_description for details. -->
   <arg name="config" default="$(optenv RIDGEBACK_CONFIG base)" />
@@ -48,7 +54,7 @@
 
   <!-- Spawn Ridgeback -->
   <node name="urdf_spawner" pkg="gazebo_ros" type="spawn_model"
-        args="-urdf -model ridgeback -param robot_description -x 0 -y 0 -z 0.1" />
+        args="-urdf -model ridgeback -param robot_description -x $(arg x) -y $(arg y) -z $(arg z) -Y $(arg yaw)" />
   <!--<node name="urdf_spawner" pkg="gazebo_ros" type="spawn_model"
         args="-sdf -model ridgeback -file /Users/mikepurvis/ridgeback_ws/ridgeback.sdf -x 0 -y 0 -z 0.1" /> -->
 </launch>