Merge remote-tracking branch 'jlack1987/feature/fix_thruster_axis'

# Updated to use ignition::math::Vector3d
# Added axis arg to Thruster
# Conflicts:
#	uuv_gazebo_plugins/uuv_gazebo_plugins/src/ThrusterPlugin.cc

Signed-off-by: Will Baker <willcbaker@gmail.com>