Attempt graceful unicorn restart

:fork_and_knife: 
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Attempt at getting runit to gracefully restart unicorn.

So this isn't exactly working in that [my crude spec to test this out](https://github.com/opscode-cookbooks/supermarket/compare/graceful-unicorn?expand=1#diff-72cc89f73c155bcd1eb4d64d6f3d59eeR13) isn't passing, returns a 500 Bad Gateway meaning Unicorn isn't restarting so gracefully. The implementation I have here is based on [this](https://gist.github.com/czarneckid/4639793) gist. Looking at the unicorn runit log everything appears to be going well... All that being said I think I'm at my limit in terms of knowledge of Unix process management and was hoping to get a push in the right direction here. @jtimberman Do you have any thoughts here? @cwebberOps mentioned that you might be able to help in this area.

Closes https://github.com/opscode/supermarket/issues/638