[limb] Add IK method

[130s]

Merely a method-ization of a sample in http://sdk.rethinkrobotics.com/wiki/IK_Ser…vice_-_Code_Walkthrough

[130s]

Example usage: While baxter_world.launch is running,

$ ipython
:
In [1]: from baxter_interface.limb import Limb
In [2]: import rospy
In [3]: rospy.init_node('baxter_limb')
In [5]: from geometry_msgs.msg import Pose, Point, PoseStamped, Quaternion
In [6]: pose = Pose(
                position=Point(
                    x=0.657579481614,
                    y=0.851981417433,
                    z=0.0388352386502,
                ),
                orientation=Quaternion(
                    x=-0.366894936773,
                    y=0.885980397775,
                    z=0.108155782462,
                    w=0.262162481772,
                )
)
In [8]: l = Limb('left')
In [9]: l.ik(pose)
SUCCESS - Valid Joint Solution Found:
[INFO] [WallTime: 1483757854.633608] [4312.677000] IK result:
Out[9]:
{'left_e0': 2.364365552432951,
 'left_e1': 1.6132818021698665,
 'left_s0': -0.5247314068347426,
 'left_s1': 0.7952608492744296,
 'left_w0': -2.7610159637242404,
 'left_w1': 1.5973748950724531,
 'left_w2': 0.37670722936273954}