Attempt at multiarch support (#545)

This is an attempt at multiarch support (#545), supercedes
(#604). The dpkg-architecture command is used to identify
an appropriate lib folder suffix to use for LD_LIBRARY_PATH
and PKG_CONFIG_PATH. builder.py and
catkin_generate_environment.cmake are modified
accordingly, ~~though there's still a problem with isolated
devel spaces~~.

~~It works for `catkin_make_isolated --install`, but I'm getting the following error when running `catkin_make_isolated`:~~


@wjwwood