Add tests for string params

Add tests for StringCommandParameter and StringParameterBuilder.
Yes, this way I assure, that stringParameter can be resolved. The bug I fixed (http://teampulse.telerik.com/view#item/279466 ) was related exactly to this scenario - the constructor of StringCommandParameter was parameterized and it was unable to resolve it via constructor injection. So with this command and its usage I assure such problem will not be raised in the future.