Add recurse keyword to array_keys and arrays

[jrbourbeau]

This PR adds a recurse parameter to the Group.array_keys and Group.arrays methods. To maintain backwards compatibility, recurse is False by default. Closes #260

TODO:

• Add unit tests and/or doctests in docstrings
• Add docstrings and API docs for any new/modified user-facing classes and functions
• New/modified features documented in docs/tutorial.rst
• Changes documented in docs/release.rst
• Docs build locally (e.g., run tox -e docs)
• AppVeyor and Travis CI passes
• Test coverage is 100% (Coveralls passes)

[jakirkham]

Thanks James! 😄