Add dep package management

[bscott]

The Go team recently added dep as a runner up for the Official package management tool, be nice to add it.

https://github.com/golang/dep

[bscott]

PR #196 already in progress

[freeformz]

Done and in master, but not yet released on Heroku. Until it's released (a few weeks) it's usable by pinning the buildpack to the master branch vs. the released heroku/go version.

Thanks!