Find galaxy_root in ~/.planemo.yml if unspecified on command-line.

Will use this value before checking parent directories (not sure if that is the right order of precedence).
Cleanup galaxy_config.py.

Make more clear module public interface is simpley galaxy_config method. Re-order methods to "Clean Code" standard.
Documentation improvements.

Mention test report in README.rst and add documentation about ~/.planemo.yml.