Fix/overloading fully qualified names

Hey!

I caught a bug yesterday when overloading classes.

If the overloading was done with `Mockery::mock('overload:\MyNamespace\Class')`, PHP would trigger a somewhat criptic error:

`PHP Fatal error:  Undefined constant 'MyNamespace' in /home/robert/www/mockery/library/Mockery/Loader/EvalLoader.php(16) : eval()'d code on line 21`

This fix left trims the backslash from the namespace name to generate valid PHP code.

P.S.: New code looks sweet! It was very easy to find and fix the bug :)