closes #24

* 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

closes #9

allow only a basic set of tags, and remove any non-mf2 classes

closes #2

sanitize the HTML returned in the content property. allows a common set of HTML tags.

for #2

if a property such as `in-reply-to` is an h-cite, the URL is still returned as the `in-reply-to` value, and the h-cite object is available in a different part of the response.

closes #6

* extracts mf2 post contents from pages
* implements authorship discovery to find author info for the URL