Add custom stash messages, and display them in /stashes

This addresses #96 and starts to address #155 . I'll send a PR into sensu-cli so that it sets source to sensu-cli (or the hostname) and allow a reason to be sent.

![silence](https://cloud.githubusercontent.com/assets/465717/4616693/46a0c524-52f6-11e4-8fe3-edf56286a247.png)

![stash](https://cloud.githubusercontent.com/assets/465717/4616704/64a56a7a-52f6-11e4-8347-35c0451a5dce.png)