* closes#23
* major refactor of the methods for extracting properties to consolidate the logic
* hReview parsing is incomplete due to issues with the php-mf2 backcompat parsing. see https://github.com/indieweb/php-mf2/issues/107
* fields that should be URLs will now be omitted if the value was not a URL, such as when the value is `javascript:alert()`
* makes Mf2 class slightly more self-contained by duplicating the URL helper functions into it
* fixes tests to not cache responses in memcache