Removed abstract from before class declaration

In cases where someone needs to extend multiple classes, they can use the following method to achieve this, but it won't work on abstract classes.

http://stackoverflow.com/questions/356128/can-i-extend-a-class-using-more-than-1-class-in-php/356431#356431

This is something fairly common when you're attempting to be extended by a base model, other packages will want to do the same, but the user is stuck on using a single package unless they use this method.

Since there is nothing abstract in the class that requires the whole class to be abstract, I've removed `abstract` from in front of the class declaration.