Do not exit if Consul is not running

- Fixes GH-103
Allow timeout to be set programatically in Watcher
Accept a -retry flag
Lock when setting the timeout function
Make a call to Agent().Self()
Do not propogate watcher errors to the parent ErrCh
Update README with retry documentation
Update CHANGELOG for retry functionality
Move that back where it goes
Fix README spacing
Make retry timeout configurable in the Config
Merge pull request #120 from hashicorp/sethvargo/retry

Support (configurable) retries