Implemented Repository Sets in CLI

Added test for subscribtions.

Requires manifest containing specific repository to work, in this case:
'Red Hat Enterprise Virtualization Agents for RHEL 6 Workstation RPMs x86_64 6Workstation'

Works with this manifest:

http://pastebin.test.redhat.com/231686

Formatting nitpick, but - is there any reason for the enormous amount of whitespace here? It definitely goes against convention, and IMO it makes this code harder to read. How about something like this?

```python
    @classmethod
    def enable(cls, options):
        """Enables a repository."""
        cls.command_sub = "enable"
        result = cls.execute(cls._construct_command(options), expect_csv=True)
        return result
```

Similar comments apply to many places in this module.