Use godep to manage dependencies

Use godep to manage/vendor dependendencies. This makes builds more reproducible and less ad-hoc than the old vendoring process.

Building is now achieved by doing:

```shell
git clone git://github.com/newrelic-forks/dogestry ${GOPATH}/src/github.com/newrelic-forks/dogestry
cd ${GOPATH}/src/github.com/newrelic-forks/dogestry/dogestry && godep go build .
```