First pass at creating a 'coauthor' user model, or a way of assigning bylines without creating WordPress users.

Very incomplete. This is what's known to work:
* Create a Co-Author with the fields (e.g. display_name, first_name, user_email, etc.) you'd expect of a normal WP user
* Assign a Co-Author to a post as the primary or secondary author
* See all posts by a Co-Author from the Manage Posts screen
* See all published posts by a Co-Author on the frontend

This is a short list from a very long list of what needs to be improved:
* Save more co-author data to the term description field
* ... so that AJAX search can search against more fields than the "Unique Slug"
* Not sure what happens if you change the "Unique Slug" for a Co-Author. Things probably break.
* Ability to associate a Co-Author with a WordPress user
* Allow for sexier unique slugs that aren't brittle with the 'cap-' prefix
* Properly update published posts count for Co-Authors
* Allow editing a co-author from the All Co-Authors view (and other improvement to that view)

See #6
Rebrand "Co-Authors" as "Guest Authors"

See #6
Rudimentary avatar support for Guest Authors leveraging featured images

This needs improvement in a couple of regards:
* Support for multiple avatar image sizes. I'm thinking we should register a few of the common ones.
* Reverse lookups against postmeta are bad. We should have a better way of seeing whether the `$id_or_email` value needs to be overridden

See #24
Resolve notices by adding a nonce check
Allow a site admin to one-click create a guest author profile for an existing WP User.

This is missing a couple of things:
* Caps check for determining whether the user has permissions to create and edit the model
* Copy all of the field data
* Finish the relationship between the WP_User and the Guest Author, slugs, etc.

See #26
Break the Guest Authors functionality into a separate class with an easy disable filter

See #6
Support for bios within the Guest Author model

See #6
Bug fix: Do the guest authors after initializing taxonomy so we have access to it

See #6
More improvements around using the taxonomy as the joining relationship between posts and guest authors.

Specifically:
* Search for co-authors based on term description instead of term name
* Store some amount of user and guest author details in the term description for searching purposes
* Trick the plugin into auto-populating author terms with descriptions if they don't exist or have descriptions
* Populate the term description field when creating a guest author from user, or updating a guest author

Slugs will be handled in a slightly different way, functionality tk.

See #6
Until we have the proper code for managing slugs, slugs for guest authors created from users are restricted to user_logins, and slugs for guest authors will be the display name from the first time the guest author is fully saved

See #6
Show the proper button depending on status
Adding initial user selection capability:
 - cap check for 'user' seems not to work, remove and let default to post
 - take dropdown code from grist_authors
Guest Author /author/slug handler
- Add a request() handler to reroute guest authors; would also work fine for user-based coauthors since it uses the taxonomy system only.
- The tag name should be the nice display name, otherwise for guest authors the page title looks like first-last instead of First Last - maybe we can fix this somehow?
Further changes to co-authors plus guest authors
- Allow modifications of 'Guest Author' label so NYO can use 'Byline' instead
- Logged-in users with a guest-author assigned should be able to create a new post and have it default to that guest-author
- Sites can force co-authors plus to only operate in guest-author mode, so no users can be set as authors.
- Additional fixes per Daniel's comments
Merge branch 'feature-authors' of https://github.com/netaustin/Co-Authors-Plus into netaustin-feature-authors

Conflicts:
	php/class-coauthors-guest-authors.php
Add all of the labels to the {{{$this->labels}}} class variable so they can be properly translated and filtered

See #31
Text changes

See #31
New helper function to determine whether the guest authors functionality is enabled

See #31
wp_get_current_user() produces a WP_User object, not an ID

See #31
When interpreting an author query, always check to see if a term exists and modify the query accordingly.

The regex will need to be modified to remove the post_author aspect if 'coauthors_guest_authors_force' is set to true

See #31, #32