main classes changed to be not abstract

is there a special reason that you declare the main classes (Client, Server and Tivoka) as abstract? according your manual these classes have to be used directly. 

if declace them as abstract, the classes cannot be instanciated:
`Fatal error: Cannot instantiate abstract class Tivoka\Client`

this prevents it from beeing easily used in the [dependency injection container][3] as [factory class][1] in the [symfony][2] framework.

[1]: http://symfony.com/doc/current/components/dependency_injection/factories.html#passing-arguments-to-the-factory-method
[2]: http://symfony.com
[3]: http://symfony.com/doc/current/book/service_container.html