Use CRUD naming in Media and CustomPostType

`WP_JSON_Posts` (and friends) was [recently](https://github.com/WP-API/WP-API/pull/546) updated to match the CRUD pattern.
Though, `WP_JSON_Media` and `WP_JSON_CustomPostType` missed the change.
Because those classes extend `WP_JSON_Posts` (which no longer has
`get_posts`, `get_post`, `edit_post`, or `delete_post` defined), fatals
are thrown. This fix renames the `WP_JSON_Media` and `WP_JSON_CustomPostType` 
method names to match the parent class.