Use interfaces from separate package

[antonkomarev]

Love for Laravel was designed with the idea in mind that it should be built on top of the framework agnostic interfaces. For now they are living in Cog\Contracts\Love namespace and located in ./contracts directory in repository root.

Contracts directory extracted to php-i/love repository with new I\Love namespace.

In next major release I want to start using external contracts instead of local ones.

This change should fix violation of the Interface Segregation Principle: php-i/love#1