Adding support for DFP async and enhancing admin UI

The plugin can now be used without any code required in the theme. The Doubleclick For Publishers Async provider includes more options in the admin to insert ads at common theme hooks, like 'before', 'before_header', and 'the_content'. The user can now choose their provider from a drop-down list on the admin page.
Bump works up to number
Restore `url_vars` as the name of the variable. Changing it to `vars` would break backwards compat.

See https://github.com/Automattic/Ad-Code-Manager/pull/52#discussion_r3017175
Revert vars to url_vars. Two more spots I missed.

See https://github.com/Automattic/Ad-Code-Manager/pull/52#discussion_r3017175
Restore tests directory
Refactor #52 with easier, functional support for DFP Async.

Some key ideas:
* DFP Async default configuration lives in doubleclick-for-publishers-async.php. The goal is for it to be comprehensive enough that all you need to do is do_action() for your ad tags. The last things to do are standardize the ad code configuration variables and default ad units.
* 'enable_ui_mapping' is a new flag you can add to your ad tags. Functionally, it means a given ad code mapped through the admin UI to an ad tag will only be registered with that tag.
* get_matching_ad_code() is a new class-level method to get the currently matching ad code for a tag. It's an abstract
ion of the conditional logic in action_acm_tag() with nothing additional added.
* Added support for `<select>` type URL vars in Quick Edit. @aliso had already added this for the "New Ad Code" form.
Revert code associated with selecting a provider from the admin.

For Enterprise sites, only Google DFP async will be supported to start. We won't need to present the user with a picker. For other sites doing the configuration at the code level, they probably don't want their users mucking with the option.
Enabling single request and collapsing empty divs if ad is not trafficked right
DFP Async: Refer to this version of tag IDs, as it will reflect what will actually have been filtered
Rename configuration variables to better match what you'd expect to receive.
Merge pull request #52 from Automattic/dfp-async

Adding support for DFP async and enhancing admin UI