AS-382, AS-404: Getting load balancers in structured form

Getting load balancer nodes and time they were drained from feeds using effect.
I think it would also be useful to document what you're actually returning.

However, before we document that, maybe we should talk about what it ought to be; I don't understand why this is a UNIX timestamp and not a `datetime.datetime`. I understand that *this* return type is a float because its result is plugged in to `LBNode.drained_at` (also a float), but that just moves the question; why is `LBNode.drained_at` a float and not a `datetime.datetime`? (I'm not sure if you're the appropriate person to ask, [but a quick check of the commit history suggests I should talk to you](https://github.com/rackerlabs/otter/commit/1b2cf0dd4be7f81f086c4c8fefd1c18e14329e6c), and [a quick check of the PR history suggests I should check @cyli and you] :smile:)

It seems to me like we're doing a little dance (`calendar.timegm`) in order to get a less useful type (when we already had a perfectly servicable type that actually represents a timestamp.

This change was pulled in on [PR#724](https://github.com/rackerlabs/otter/pull/724/files), but I can't find any comments on it there. That's strange, because I could've sworn I commented on [this line](https://github.com/rackerlabs/otter/pull/724/files#diff-214e5deb5d9b8642d733dba6e0bc3672R198), very specifically about "EPOCH". Do you recall any such comment?

It appears some comments by @cyli made it through, though. Looks like the reason is that it used to be a relative timestamp, and then became an absolute one, but the type wasn't changed? ([discussion](https://github.com/rackerlabs/otter/pull/724#discussion_r19188246))