Adding tests for Account.add_group().

These tests ensure we can accept the group as either:

- An object.
- A href.
- A group name.

This makes it easier for developers to add groups.
Modifying Account.add_group() method.

It now accepts groups as a href, object, or name.
Merge branch 'feature/object_or_href_or_string' into develop

* feature/object_or_href_or_string:
  Modifying Account.add_group() method.
  Adding tests for Account.add_group().

This commit adds flexibility to the Account.add_group() method.

It allows developers to specify the group as an object, href string, or group
name.  This makes it much easier for developers to work with, since they need to
do much less work upfront.