Implements massless position controlled control surfaces

Works on #107 by implementing position controlled control surfaces via linearizing around u = 0 to create a B matrix for the control surfaces.  Includes a unit test and, while changing a substantial amount under the hood, does not modify the existing RigidBodyWing interface and testWing should still pass.

I did this by implementing RigidBodySubWing classes that dealt with the actual computation and a RigidBodyWing (which can contain multiple subwings) sums up the forces created by its subwings.  The old interface is implemented by creating a single subwing and having calls basically pass right through RigidBodyWing to its instance of the RigidBodySubWing.  @blandry should take a look.
it's ok to not write up the contact geometry (if it's not implemented)

On Oct 24, 2014, at 6:07 PM, Andy Barry <notifications@github.com> wrote:

> In doc/drakeURDF.xsd:
> 
> > +        <xs:annotation>
> > +            <xs:documentation>Width of the control surface.</xs:documentation>
> > +        </xs:annotation>
> > +        <xs:attribute name="left_edge_position_along_wing" type="xs:double" use="required" />
> > +        <xs:annotation>
> > +            <xs:documentation>Position of the left edge of the control surface along the wing (in meters).</xs:documentation>
> > +        </xs:annotation>
> > +        <xs:attribute name="min_deflection" type="xs:double" use="required" />
> > +        <xs:annotation>
> > +            <xs:documentation>Minimum control surface deflection in radians.  Typical is around -0.90</xs:documentation>
> > +        </xs:annotation>
> > +        <xs:attribute name="max_deflection" type="xs:double" use="required" />
> > +        <xs:annotation>
> > +            <xs:documentation>Maximum control surface deflection in radians.  Typical is around 0.90</xs:documentation>
> > +        </xs:annotation>
> > +    </xs:complexType>
> >      <xs:complexType name="thrust">
> Added, except for the contact geometry which isn't implemented yet, so I didn't want to clutter up the URDF side with it. If you think it's important, I can implement it.
> 
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