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`:
~~~
==> Generating an env.sh
Unhandled exception of type 'OSError':
Traceback (most recent call last):
  File "./src/catkin/bin/../python/catkin/builder.py", line 849, in build_workspace_isolated
    number=index + 1, of=len(ordered_packages)
  File "./src/catkin/bin/../python/catkin/builder.py", line 612, in build_package
    destdir=destdir
  File "./src/catkin/bin/../python/catkin/builder.py", line 527, in build_cmake_package
    arch = get_multiarch(build_dir)
  File "./src/catkin/bin/../python/catkin/builder.py", line 253, in get_multiarch
    return run_command('dpkg-architecture -qDEB_HOST_MULTIARCH 2> /dev/null', build_dir, quiet=True)
  File "./src/catkin/bin/../python/catkin/builder.py", line 188, in run_command
    raise OSError("Failed command '%s': %s" % (cmd, e))
OSError: Failed command 'dpkg-architecture -qDEB_HOST_MULTIARCH 2> /dev/null': [Errno 2] No such file or directory
<== Failed to process package 'sdformat': 
  Failed command 'dpkg-architecture -qDEB_HOST_MULTIARCH 2> /dev/null': [Errno 2] No such file or directory
Command failed, exiting.
~~~

@wjwwood