implement lookup plugins for arbitrary enumeration over arbitrary things.   See the mailing list for some cool examples.

Adds a thirsty=yes|no to the get_url module, such that if downloading a large file from the internet you can decide
whether to download it every time or not -- will replace only on change, or decide to not download.  The default
is thirsty=no which will not download every time by default.

Adds the 'serial' keyword to a playbook which controls how many hosts can be running through a playbook at a single time.
The default is 0, which means all hosts.  If set to 1, each host would run a playbook all the way through before moving
on the next host.  Fact gathering is still parallel, regardless of the serial setting.

If ANSIBLE_KEEP_REMOTE_FILES=1 is set as an environment file, remote files will not be deleted, which is useful only for development debugging purposes.