Take statsd host and port from environment if missing in the config.

[porridge]

Fixes: #28

[porridge]

I manually tested that:

• omitting the settings from both environment and config properties falls back to localhost:8125
• omitting settings from config properties but providing them in the environment picks up the latter

Please let me know whether I should bump version (and to what, since master seem to be 0.5.2 but there is some mysterious 0.5.3 at https://bintray.com/airbnb/jars/kafka-statsd-metrics2/0.5.3#files/com%2Fairbnb%2Fkafka-statsd-metrics2%2F0.5.3).

[porridge]

friendly ping, anyone? @ziliangpeng ?

[porridge]

friendly ping again, @ziliangpeng @alexism @parasitew