catkin_make_isolated now generates environment setup files without source packages

See http://answers.ros.org/question/62901/should-catkin_make_isolated-generate-some-initial-setup-files-without-packages-in-src/ for context.

This patch includes the following changes:
1) Adds setup environment files in devel_isolated. This behavior should only change iff there are no packages in the source directory.
2) Minor bug fix inside get_python_path inside catkin builder.py (line 228)