Offline support

Also changed (uhh…whoops)

- Maximum valid new metric age
- Created concept of metric collections (for "counters" and "gauges")
- Changed default data passed around from `NSDictionary` data to `LibratoMetric` that is only converted to JSON immediately before submitting to Librato.
- Queue is now flushed after submission (whoops)
- Queue is now smarter about how to handle random things thrown at `add:`
- Queue is now a bit worse about how it handles random things thrown at `add:`
- Processor/Queue, Collection and Metrics all now inherit from [Mantle](https://github.com/github/Mantle) for automatic support of `NSKeyedArchiver` and better obj->JSON and JSON-obj transformation.