Added methods to allow the cleaner to delete isolated networks

* This relies on the user having an up-to-date version of cloud/rax
  cafe installed as it is using a recent addition to the cafe
* cafe-cleaner will now attempt to delete any isolated networks
  after other resources have been cleared. This is neccesary as
  networks attached to still-existing servers will not delete.
* When using this it is possible that servers may take longer than
  expected to delete. In this case the cleaner may need to be run
  again later to catch any networks that are still hanging about.
Merge pull request #1 from ananci/network_cleaning

Added methods to allow the cleaner to delete isolated networks