JSON: return list of items (closes #10)

As per
http://www.whatwg.org/specs/web-apps/current-work/multipage/microdata.html#json
we should return a dictionary with a key "items" containing a list of
the extracted items.
JSON: use "properties" container for item props

As per
http://www.whatwg.org/specs/web-apps/current-work/multipage/microdata.html#json:

“Add an entry to result called "properties" whose value is the object
properties.”

Closes #13
PEP-8

* Import tidying
* Strip trailing whitespace
* Two-line separation of top-level functions
JSON: convert itemtype to list (closes #11)

As per http://www.whatwg.org/specs/web-apps/current-work/multipage/microdata.html#attr-itemtype itemtype should be split on spaces into
a list
Merge branch 'item-json-structure' of git://github.com/acdha/microdata
small fixes to update tests and documentation to match the latest behavior.