Fix etcd reporter

[bitglue]

The existing implementation was quite clearly a half-baked copy & paste of the zookeeper watcher with s/zookeeper/etcd. Update it to actually work.

• make the log messages sensical
• fix ttl undefined name error
• don't create a new key on each check_interval
• clarify the more odd parts of the code with comments
• rescue exceptions that actually exist in Etcd
• don't rescue exceptions when it's not necessary: etcd automatically creates directories
• calculate a TTL based on the check_interval

Supersedes #55.

CONTRIBUTING.md requests pull requests to be made into the pull_requests branch, but that branch is a year old, so I'm requesting a merge to master.

[philips]

lgtm

[bitglue]

@igor47 any comments?

[igor47]

looks great, thanks for the PR!