Merge pull request #82 from prometheus/julius-fix-targets

Fix target pool deadlock and health checking bugs
Fix target health state update.

Right now, futureState is only used to give hints to the health scheduler, but
nowhere is this future state persisted into the target's state field, so we
don't actually track a target's state over time.
Merge pull request #85 from prometheus/julius-fix-healthstate

Fix target health state update.
Allow replacing job targets via HTTP API.

This roughly comprises the following changes:

- index target pools by job instead of scrape interval
- make targets within a pool exchangable while preserving existing
  health state for targets
- allow exchanging targets via HTTP API (PUT)
- show target lists in /status (experimental, for own debug use)
Update README.md
Merge pull request #86 from prometheus/julius-sd

Allow replacing job targets via HTTP API.
Fix compile error in metric helper function.
Fix typo in comment.
Implement remaining View Get* methods.
Merge branch 'master' into refactor/storage/batching