Fix relative path to only test xylem source directory.
Fix test_code_format to use flake8 and correct directory.
Fix various code style issues.

 - Now passes flake8
 - Maximum text width 79
 - Maximum text width 72 for multiline-comments and -docstrings and text files.
 - First line of docstring should use imperative and end with '.'
 - Added some docstrings, fixed some typos.
Merge pull request #5 from catkin/tests

Fix relative path to only test xylem source directory.