Load custom adapters

Paywall can isntall custom Adapters from now on. These adapters can be places in any plugin but under the specified directory: `plugins/ExamplePluginBundle/Adapter/PaywallAdapter/`.

Adapter should have `Adapter` text at the end of adapter name, e.g. `MembershipAdapter.php`.

In this example `MembershipAdapter.php` must be archived to `MembershipAdapter.zip`.

Paywall will automatically install it during the installation process or update process.