Merges #24: QuotableMixin.published can now be overriden - thanks @garrettc.

Fixes #26: requests 2.20 or greater (until version 3) are now allowed. Likewise for beautifulsoup4 4.6 and mf2py 1.1.

Added get_mentions_for_view(HttpRequest) -> Iterable[QuotableMixin] convenience method. This may be used to retrieve mentions for rendering directly in a Django template, as an alternative to using the webmention/get endpoint from a frontend script.