Added LAS reader plugin.

It uses libLas as a 3rd party library. Internally, it requires BOOST.
- The COLLADA_INCLUDE_DIR points to the right place, but a new CMake var should be added.
- The BoostThreads library is included in the full 3rd party package (http://www.openscenegraph.org/index.php/download-section/dependencies )

The plugin is a mix between the 3dc plugin and the libLas read example.

The plugin has a "v" option to show the LAS header during the parsing.