fix for Machine/Task Correlation when performing a db.fetch

This fix partially solves issue #302 (solves only the machine correlation and not the tag correlation)

Description of the change:
In lib/cuckoo/core/database.py: 
1. added an optional 'machine' argument for the fetch function - if supplied , the fetching will be done only for this machine
2. added a get_available_machines function - returns all available machines
In lib/cuckoo/core/scheduler.py:
1. in the 'start' function - fetching is done in a loop to fetch tasks for each available machine seperately

Note: This code was written in Check Point Software Technologies LTD.