Add autoloading support for standalone_migrations gem

I created a [pull request](https://github.com/thuss/standalone-migrations/pull/100) for the [standalone_migrations gem](https://github.com/thuss/standalone-migrations).  That pull request added support for an on_load callback.

This pull request is to add the ability for foreigner to be properly loaded (`Foreigner.load`) when somebody is using the standalone_migrations gem.  Similar functionality is provided for rails.

I also added tests to ensure that the foreigner autoloaders for both rails and standalone_migrations are loaded only if the respective library is loaded.  If `on_load` is not supported with standalone_migrations, this gem will not be loaded.