Gracefull exit on sigTERM

Like the sigINT that was already handled, postgrest
should gracefuly shut down on a sigTERM. This is a
common way of stopping processes, used amongst
others by docker.

See: https://stackoverflow.com/questions/4042201/how-does-sigint-relate-to-the-other-termination-signals

CHANGELOG.md

@@ -6,6 +6,7 @@ This project adheres to [Semantic Versioning](http://semver.org/).
 ## Unreleased
 
 ### Fixed
+- Terminate gracefully on SIGTERM - @recmo
 
 ## [0.3.1.0] - 2016-02-28