Port Entity Reference Module

https://github.com/backdrop/backdrop-issues/issues/51

This implements a very rudimentary plugin system using the familiar hook_x_info and drupal_alter pattern, but skiping the work of altering in the default case until a module declares that it needs to alter. Initial performance test shows this to be one or 2 orders of magnitude faster than always altering. I'm assuming 80% of developers will never add these kinds of plugins, because Backdrop wont convert everything is possibly can to the plugin system.