Feature resource properties

The `Property` class describes a property, which has a type and a target. 

The `Searchable` class represents (no way) the searchable property.

Fixes #2.

@JBlaak 
http://php.net/manual/en/function.key.php since we have only one key it is not obligatory to `reset` the array first. But if you want to be sure you can do that as well