(TK-92) Add graceful-shutdown support

Add graceful-shutdown support to the Jetty9 Webservice. Now,
when the webserver is shutdown, the server will give open
requests a chance to resolve themselves. Allow user to
configure the graceful shutdown stop timeout.
(TK-92) Address PR comments

Change timeout setting to `shutdown-timeout-seconds`.
Reduce sleep times in graceful shutdown tests.
Don't do graceful shutdown when stop timeout is set to 0 seconds.
Merge pull request #88 from fpringvaldsen/TK-92-graceful-shutdown

(TK-92) Add graceful-shutdown support
Update CHANGELOG and version for 0.8.0 release
lein-release plugin: preparing 0.8.0 release
lein-release plugin: bumped version from 0.8.0 to 0.8.1-SNAPSHOT for next development cycle
(TK-95) Ensure default timeout is 60 seconds

Fix issue wherein the default shutdown timeout was 30 seconds
rather than 60 seconds.
Merge pull request #89 from fpringvaldsen/TK-95-timeout-fix

(TK-95) Ensure default timeout is 60 seconds