Changed bind and unbind commands to use the new PollingCommand model

Like the others that derive from PollingCommand, the bulk of the neat stuff is in the poll() command, so the changes in the bind and unbind commands is mostly stripping out stuff.

The difference here from the others I've done already is that bind/unbind produce multiple tasks. I added hooks in the PollingCommand when I wrote it to display a header before each task to describe what was being tracked, that way it's not just a bunch of random spinners. These subclasses use that hook to display the steps of each operation. That will be more clear when I post the screenshots. There might be a slightly different way of handling these where more of the switching logic is in the base class, but I'm not entirely sure yet and it's so small I'm not in a huge rush to force it in.

As part of those headers, I changed the bind/unbind iteneraries to use constants for the action tags. Another minor tweak in here is the phrasing change from "Request Successful" to "Task Successful".

There were no unit tests for either the bind or unbind command so I added those as well.

There are no changes to either pulp_rpm or pulp_puppet, all of the functionality is in the base class.