Add `get_instances()` to `WP_JSON_User_Response`

Also adds basic tests
Make sure to pass context to `get()`
Retool tests so current user context is more explicit

Also adds tests where the request is a logged-out request
Allow `role` to be specified when creating / editing

See #335, #392
Return `WP_Error` when user is invalid

We're checking for `WP_Error` in more places than falsy
See if this fixes the PHP 5.2 error
Merge branch 'two-dot-oh' into more-user-resource

Conflicts:
	lib/class-wp-json-users.php
Merge pull request #427 from WP-API/more-user-resource

Move more into WP_JSON_User_Resource