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.
Cleaning up the classifiers stuff.

This makes our setup.py script a bit nicer, as it's a bit simpler.
Using the 'six' library to handle python3 strings properly.

Backporting the changes to the Account.add_group() method.
Adding tests for Account.add_group().

These tests ensure that we return an appropriate exception if the group passed
to us is invalid.
Implementing exception handling for Account.add_group().

We now properly handle raising the appropriate exception type based on user
input.
Making the 'six' dependency flexible.
Fixing conflicts.
Moving our tests over to a helper method.

This keeps things isolated.
Modifying Account.in_group() and Account.in_groups().

These now accept group objects, hrefs, or names!
Bumping readme with change details.
Adding tests for a _resolve_account helper method.
Cleaning up imports.
Adding tests for our Group.add_account() method.

This ensures the method acceps accounts in all forms.
Adding Group._resolve_account() method.

This grabs an Account resource in a number of ways:

- Object.
- href
- Username
- Email
Modifying Group.add_account().

It now supports all forms of account specifications!
Updating changelog.
Ensuring our Group.add_account() method raises an error if the user is already a
member of the group.
Adding test to ensure that if we try to add a group to an account that already
exists, we raise an exception.
Adding tests for Account.remove_group() method.
Implementing Account.remove_group().