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.
Merge latest master to ForceToActuationRename branch