Update CMakeLists to ensure it can be build/package as an extension

* Add a CMakeLists.txt to the top level as expected by the Slicer extension build system
* Use the convenience macro "slicerMacroBuildScriptedModule"
* Simplify CMakeLists.txt
* Remove extension description file from the repository since an up-to-date
one is automatically generated when building the project.

Note that the iconurl currently points to the generic Slicer icon, a custom
one would have to be provided.

A wiki page would have to be created on Slicer wiki. See http://www.slicer.org/slicerWiki/index.php/Documentation/Nightly/Developers/Tutorials/DocumentExtension#Extension_page

To generate an extension description:

$ git clone git://github.com/jcfr/SlicerWebGLExport.git
$ mkdir SlicerWebGLExport-build
$ cd SlicerWebGLExport-build
$ cmake -DSlicer_DIR:PATH=/home/jchris/Projects/Slicer-AHM-Superbuild-Debug/Slicer-build/ -DMIDAS_PACKAGE_EMAIL:STRING=<YOUR_EMAIL> -DMIDAS_PACKAGE_API_KEY:STRING=<YOUR_WEB_API_KEY> ../SlicerWebGLExport

File "WebGLExport.s4ext" has been generated. Make sure to change the scmurl
to a readonly one. See http://www.na-mic.org/Bug/view.php?id=2395

For more info about extension (get an API key, upload experimental extension, etc ..)
See http://www.slicer.org/slicerWiki/index.php/Documentation/Nightly/Developers/Tutorials/BuildTestPackageDistributeExtensions