Changed FacebookBase::errorLog to a non-static method

Changed `FacebookBase::errorLog` to a non-static method so that it may be overridden by subclasses. Although this method has been marked as protected, it was never possible to override its functionality through subclassing because references to it were made with non static binding `self::`. Rather than changing these references to `static::` and breaking compatibility with php 5.2, I've chosen to remove the static qualifier from the method.

All references have been updated from `self::errorLog(...` to `$this->errorLog(...`
The unit test `testErrorLogOverride` has been added to ensure that error logging behavior can be overridden by subclass.