Update rac.py

Added __enter__() and __exit__() functions to RAC so that the "with" keyword can be used.

import rac
hostname = 'test.local'
username = 'test'
password = 'pw'
with rac.RAC(hostname, username, password) as rac_obj:
    print rac_obj.run_command('getsysinfo')

Thanks, Micah