Librato Outputter refactor

* Backoff starts at 500ms and grows exponentially from there.
* Use http.Client's Timeout instead of man handling the underlying bits
* Clean up error handing in send / sendWithBackoff as responsibilities
  were jumbled up before.
* Test how many times retried
* Increate default timeout to 10s default
Listen Poller Refactor

* Simplify ListenStats to just use counters. Removed a bunch of code.
* Use a scanner instead of man handling a reader
Additional listen poller cleanups

* Use sync/atomic and declare counters explicitly
* Don't differentiate the parse errors, just count them all
* s/stats/_meta_/ to help ensure we won't conflict
* Don't make quite so many new fmt.Errorf's
Remove mutex, and do atomic increment instead.
Merge branch 'revisit' of https://github.com/heroku/shh into revisit

Conflicts:
	listen_poller.go
	listen_poller_test.go
Merge pull request #35 from heroku/revisit

Revisit listen_poller, librato_outputter