Features/httpinput-enhancements

- Allow for an array of URLs in HttpInput TOML configuration
- Inject Timestamp into HttpInput messages, useful for a resumed health check
- Successful HttpInput GETs default severity to 6, Unsuccessful error messages at Severity 1
- Generate internal heka message of type heka.httpdata.error with severity 1 for non-200 status code GETs and failed connections; useful for health checks.