Bashbrew

We have been working on a more transparent and simplified version of stackbrew for building and pushing using a small set of bash scripts:

- `build.sh` will `git clone`, `docker build`, and `docker tag` the specified repos and will take advantage of docker build caching by setting the mtimes of all files and directories to the date of the commit that most recently modified the file (or directory contents)
- `push.sh` just does a `docker push` on the specified repos for the namespaces (default: `stackbrew library`)
- `git-set-mtimes` is just the helper script to set the mtimes for `build.sh`

Since we use docker cache to control rebuilds, the builds can be run every night or even more often than that with no issues.  We added cascading builds (an image `FROM` another being built gets pushed to the end of the build queue, so security fixes to base images automatically propagate).  We get an extra bonus to be able to drop in Travis to test PRs to ensure that they can be cloned and have the proper library file format.