Adds support for Floating IP

A comment from the non-expert sidelines here: should the `@returns` part of the Javadoc not describe in sufficient detail what the method returns in which cases? That's always where I first look when trying to understand what a method can give back.

And if we find ourselves in a situation where we are saying "well, when the method returns `null` then it can _either_ mean that this good thing happened _or_ that this bad thing happened, and you have no way of telling the difference"...**then** I'd agree that we have a problem that we need to address.

If we can adequately map outputs to unique states/actions in the target system, I think we don't immediately need to consider massive changes. I fully agree with @nacx that it would be great if jclouds _consistently_ returned the same results for similar types of states/actions on the target system.

I suspect that implementing that will take quite some analysis _and_ a bunch of breaking changes, though, so I'd definitely want to track that in a separate PR and make sure we communicate early with all our users on this topic. 

Right now, I think a reasonable initial goal is to make sure return values are consistent _within_ and API and, if possible, a family of APIs such as OpenStack.