Joint Coulomb friction

[jslee02]

This pull request adds Coulomb friction property to joint. Figure 1 of this paper describes well about the Coulomb joint friction.

Joint Coulomb friction works only for dynamic joint types (PASSIVE, FORCE, and SERVO).

To set joint friction:

// in code
Joint* joint = skel->getJoint("my joint");
joint->setCoulombFriction(0, 10.0);

<!-- in SKEL -->
<joint type="revolute" name="my joint" actuator="force">
    <parent>world</parent>
    <child>link 1</child>
    <axis>
        <xyz>1 0 0</xyz>
        <dynamics>
            <damping>1.0</damping>
            <friction>1.0</friction>
        </dynamics>
    </axis>
</joint>

This PR fixes #293 as well.

[jslee02]

+1

[scpeters]

This would help resolve gazebo issue 1281