Changed algorithm for locating shared libraries:

 - ert/__init__.py will try to import a module __ert_lib_path which
   should contain information of where the libraries are located.

 - The ert/__ert_lib_path module is created from cmake using
   configure_file(); one is created for the build directory and
   another for the installed directory.
Made PYTHON_INSTALL_PREFIX a cmake user variable.
Removed job_queue dependency
Using lib/pythonX.Y/site-packages as default python prefix