Add enabling and disabling of members.

This is the most frequently used action when managing load balancers.

Use the `BalancerManager` class for enabling and disabling members.

    manager = BalancerManager('http://your.domain/balancer-manager')
    manager.enable('member1')
    manager.disable('member1')

You can enable or disable members from the command-line, too.

    quaidan.py http://your.domain/balancer-manager enable member1
    quaidan.py http://your.domain/balancer-manager disable member1