Process counts payload in Librato Metrics service

Handle a new JSON webhook format that contains only counts for each timeseries.


### TODO

 - [ ] Ignore "too far in the past" errors when testing webhook
 - [x] Convert to epoc dates from ISO8601
 - [ ] :shipit:
There's no way at present to know if an alert is a test or not. As discussed in chat, skipping this for now.