Add unbind command

[carolynvs]

Closes #82.

$ svcat unbind --help
Unbinds an instance. When an instance name is specified, all of its bindings are removed, otherwise use --name to remove a specific binding

Usage:
  svcat unbind INSTANCE_NAME [flags]

Examples:

  svcat unbind wordpress-mysql-instance
  svcat unbind --name wordpress-mysql-binding

Flags:
  -h, --help               help for unbind
      --name string        The name of the binding to remove
  -n, --namespace string   The namespace of the instance or binding (default "default")

[arschles]

@carolynvs rebase needed here