Remove GUI license. Rename API license.

Also updated the year in the copyright.
[license] Moving author list to CONTRIBUTING.md.
[doxygen] Fix Function.h typos

It's possible that the description isn't correct (about returning NaN). I didn't look into this.
Merge pull request #204 from opensim-org/function-doxygen-typo

[doxygen] Fix Function.h typos
Merge pull request #190 from opensim-org/license

Remove GUI license. Rename API license.
[doxygen] AbstractTool.h typo.
Merge pull request #205 from opensim-org/abstracttool-typo

[doxygen] AbstractTool.h typo.
Remove unnecessary include of ControlConstant.
Merge pull request #208 from opensim-org/remove-ControlConstant-from

Remove unnecessary include of ControlConstant in Manager.
Include PlanarJoint in osimSimulation.h.
Merge pull request #213 from opensim-org/include-planar-joint

Include PlanarJoint in osimSimulation.h.
Merge remote-tracking branch 'remotes/origin/master' into geometry
Add Appearance Property to Geometry to serve as dfaultAppearance.