added FindGSL and updated README.md accordingly

The .cmake file was taken from: http://sourceforge.net/p/openmodeller/svn/HEAD/tree/trunk/openmodeller/cmake/FindGSL.cmake
The original GSL code does not provide a .cmake file. It would be useful to include this here.
This should be `GSL_INCLUDE_DIRS`, even if the path is singular. See:

https://github.com/ros/cmake_modules/blob/0.3-devel/CONTRIBUTING.md#cmake-module-style