Adds support for Floating IP

Great analysis @nacx. As always I'm primarily concerned with consistency. From your analysis it's clear that the OpenStack/Rackspace stuff is consistently inconsistent with how the rest of jclouds handles this kind of case. 

I appreciate the desire to make this 100% consistent across all of jclouds but look at it from the perspective of a user of the OpenStack/Rackspace code. Absolutely everywhere else they know have to handle null, but in this one little API off in the corner they have to handle a ResourceNotFoundException (and probably won't even be aware they need to do that). Now the OpenStack/Rackspace stuff is inconsistently inconsistent with how the rest of jclouds handles this kind of case. To me that's a worse place to put our users.

I would rather see this PR be consistent with all of the other OpenStack/Rackspace code and then open a JIRA issue to consider making a complete switch of OpenStack/Rackspace to using ResourceNotFoundException.

Thoughts?