Add orderby filtering to REST API resource endpoints

These commits add orderby filtering to the REST API resource endpoints:

* `order` filtering to GET /customers
* `orderby` filtering to all resource endpoints, as supported by WP. For coupons, orders, and products this means [WP_Query orderby params](http://codex.wordpress.org/Class_Reference/WP_Query#Order_.26_Orderby_Parameters) and for customers this means [WP_User_Query orderby params](http://codex.wordpress.org/Class_Reference/WP_User_Query#Order_.26_Orderby_Parameters)
* `meta_value`/`meta_value_num` orderby parameters are supported by an additional filter param, `filter[orderby_meta_key]`

Closes #5720