Adds More Server-Side Filtering Options for CCI listing.

This is aimed to resolve #72

Examples:

    $ sl cci list --datacenter=dal05
    $ sl cci list --network=100 --cpu=2
    $ sl cci list --memory='>= 2048'
    $ sl cci list --tags=production,db

The CCI manager has more filter options added to it.

    CCIManager.list_instances(
        self,
        hourly=True,
        monthly=True,
        tags=None,
        cpus=None,
        memory=None,
        hostname=None,
        domain=None,
        local_disk=None,
        datacenter=None,
        nic_speed=None,
        **kwargs)

Related changes:
* CLI: Moves NestedDict to top-level utils module so the managers can use it.
* CLI: Adds to_dict() method to NestedDict since the XML-RPC library is too strict about types.