Add some comments to the examples

Add some useful usage comments to the examples.

Change-Id: Id12ba3883492d69f971b78c75048fc7f5914998d
Allow --data option to pass UUID instead of json

Allow the user to pass UUID only instead of full json.  This will
be handy for get and delete.  For example:

    python -m examples.get openstack/network/v2/network.py \
           --data ab13c9e7-3f30-4eee-9f96-41deb0223183

Change-Id: I12f15728b908d21110e9dead857d24d903bf17a9
Add example script for HEAD requests

Change-Id: I3d2421f1fb6e0db02e01789edc78ca432e34975f
Don't join None values in utils.urljoin

This was seen on resources that don't set an id attribute, ending up
with request URLs such as /v1/blargablarga/None instead of
/v1/blargablarga/.

Change-Id: Ib6fa52c6d4b76f6c9d30418277fdd927afb5d102
Merge "Add some comments to the examples"
Merge "Add example script for HEAD requests"
Merge "Allow --data option to pass UUID instead of json"
Merge "Don't join None values in utils.urljoin"