remove legacy support for XRD and host-meta

This removes support for host-meta as well as the XRD version of
webfinger documents.  Other plugins can still support alternate
representations of the webfinger resource using the 'webfinger_render'
action.  This action has been simplified to take only the webfinger data
array, since there shouldn't be anything user-specific in the rendering,
and the HTTP Accept header can be retrieved from $_SERVER if needed.

The default behavior will be to render the webfinger resource as a JRD,
regardless of the HTTP Accept header.  This is consistent with the
WebFinger specs instruction to ignore unsupported representations.

This commit also switches to use WordPress's status_header() function.

fixes #1
Merge pull request #3 from willnorris/format

remove legacy support for XRD and host-meta
updated version number

The update is so radical that it should be a major update and I updated
the changelog and added an "Upgrade Notice"
added @willnorris as a contributor
prefixed the resources-filter
complete refactoring of the get_user_by_uri function
removed some whitespaces and added more descriptive var-names
fixed some more whitespaces
added a filter to change user
Merge pull request #8 from pfefferle/refactor-get-user-by-uri

I had a look at WP_Query but it seems that you need the query params and that is exactly why I implemented the url_to_authorid function. When using modrewrite we don't have the query-string.
consistent subject value

fixes #6
capital F (WebFinger)
whitespace stuff
removed trailing whitespaces
added support for multiple rel-values

fixed #9
simplified code
some cosmetics
renamed function to make sure it is for users only
used more descriptive filter names
new filter to ignore replace the complete query procedure