Section: dict like access for sections

We prefer properties over sections to not break any existing
code. Cf. issue #102
Section: return sections also in .items()

To be consistent with dict like access
Section: also check for subsection in __contains__

Again, to be consistent with dict like access
[Test] Section: test for dict like access
Section: return props+sections from from .__iter__
Merge pull request #106 from gicmo/section_dict_section

Section dict access: expose sections from s['key']

LGTM, thanks alot