Adding more debug information in the compile phase

After trying to release a heroku go front end that was not trivial I found myself really needing a little bit more debug information out of the buildpack.

Here's a sample output with this change:

    $ git subtree push --prefix src heroku master
    ...
    -----> Fetching custom git buildpack... done
    -----> Go app detected
    -----> Using Go 1.0.3
    -----> Setting up ENV variables
           GOROOT=/app/tmp/repo.git/.cache/go-1.0.3/go
           GOPATH=/tmp/build_2j4auww7lp5gu/.heroku/g
           PATH=/app/tmp/repo.git/.cache/go-1.0.3/go/bin:/app/tmp/repo.git/.cache/venv/bin::/usr/local/bin:/usr/local/sbin:/usr/bin:/bin:/usr/sbin:/sbin
    -----> Setting up the directory structure
           Creating directory /tmp/build_2j4auww7lp5gu/.heroku/g/src/
           Copied [Procfile, okvivi] into the GOPATH/src directory
    -----> Running: go get -tags heroku ./...
    -----> Discovering process types
           Procfile declares types -> web

    -----> Compiled slug size: 1.0MB
    -----> Launching... done, v10
           http://gofe-example.herokuapp.com deployed to Heroku

This is after pushing this Go server example that's not trivial anymore and involves a directory structure that's closer to real usage of Go, with multiple packages: https://github.com/okvivi/go-server-on-heroku