Load custom adapters

Paywall can isntall custom Adapters from now on. These adapters can be placed 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`.

Every adapter needs to be archived.
In this example `MembershipAdapter.php` must be archived to `MembershipAdapter.zip` and placed to `plugins/ExamplePluginBundle/Adapter/PaywallAdapter/MembershipAdapter.zip`

Paywall will automatically install it during the installation process or update process.

Default Adapter is `PaypalAdapter` and it will be set by default when Paywall plugin will be installed.