If the input URL contains a fragment, finds the DOM tree at that ID and runs the subtree through the mf2 parser.

closes #15

* 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

errors fetching the URL should not result in a 400 response

closes #9

use fork of php-mf2 until https://github.com/indieweb/php-mf2/pull/83 is merged

see http://microformats.org/wiki/microformats2-parsing-issues#ignore_u-camelCase_properties for background

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

file_get_contents is dumb. I hope this isn't a permanent solution.