Adding support for passing in path and class_name.

Handing additional configuration like path and class_name in the
initParams for assisting in telling the job which file and class to
load for the worker.
Adding additional error details to the error messages.

Adding the class name if the class isn’t found or if the class is not
of type Net_Gearman_Job_Common.
Better error handling

Moving the Net_Gearman_Job::factory call inside of the try block since
it can throw exceptions.
Changing PEAR_Exception type hint to Exception.

This is more compatible for non-PEAR installs.