[#67664790] implement tcp listener

Signed-off-by: Johannes Petzold <jpetzold@pivotallabs.com>
go fmt; make bin/test pass

Signed-off-by: Johannes Petzold <jpetzold@pivotallabs.com>
[#67664790] implement LoggingEmitter

- Change dropsonde-agent/emitter Emit() to use []byte instead of Event
- Add logging if emitting fails
- Implement logging-only emitter.

Signed-off-by: Johannes Petzold <jpetzold@pivotallabs.com>
[#67664790] implement main()

- create LoggingEmitter if --debug flag is set
- handle SIGINT and SIGTERM

Signed-off-by: Johannes Petzold <jpetzold@pivotallabs.com>
[#67664790] add agent start/stop log messages

Signed-off-by: Johannes Petzold <jpetzold@pivotallabs.com>