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.

add the limit option, which can be used to further confine the patterns selected by "hosts:" in ansible-playbooks
to an additional pattern (a subset) specified on the command line.  For instance, a playbook could be reusable
and target "webservers" and "dbservers", but you want to test only in the stage environment, or a few boxes at a time.

Add ability to store and access module results later on in the play.  See examples/playbooks/register_logic for details.

Only run yum,apt through the with_items loop if there is a list of items.  Just a cosmetic fix on return values
for the most part.

./group_vars/groupname.yml (for all groups the host is in)
./host_vars/hostname.yml (for the hostname given in the inventory)

This requires an actual inventory file, not script and the paths are relative
to the directory of the inventory file.

Plays and tasks now yell when they see parameters they do not understand, so that typos like 'var' for 'vars'
don't result in people having to ask questions about what is wrong with their playbook