Add support for software trigger

[mjavault]

Issue #128 - add support for software trigger for cameras that support it.
You can use it as follows:

    <node pkg="nodelet" type="nodelet" name="camera_nodelet"
          args="load pointgrey_camera_driver/PointGreyCameraNodelet camera_nodelet_manager" output="screen">
...
      <param name="frame_rate" value="15" />
      <param name="enable_trigger" value="true" />
      <param name="trigger_mode" value="mode15" />
      <param name="trigger_source" value="software" />
      <param name="trigger_parameter" value="5" />
      <param name="timeout" value="-1" />
...
    </node>

then you can publish a std_msgs/Empty message on the trigger topic (note: that topic only exists if there is at least one consumer connected to the image topics).