make relative URLs absolute

[edsu]

The URI class should probably be smart enough to absolutize URLs as property values once they are removed from the context of the HTML they came from.

[acdha]

Heh, I was just about to request this - I was comparing with @jronallo's ruby microdata library and noticed that as the most significant content difference (there are some minor differences in how embedded sub items are structured):

https://gist.github.com/acdha/5224776#file-microdata-python-json-L21
https://gist.github.com/acdha/5224776#file-microdata-ruby-json-L10

Guess I owe you another pull request ;)