Use docks to manage all texastribune docker images

This pr:

1. adds extra complexity using `git subtree` :ghost: 
2. but lets me be lazier by letting us store all our docker images in one repository so we can review each other's dockerfiles faster because we won't have to clone a new repo and set up new editor environments every time
3. makes it easier to refer to other Dockerfiles we've written
4. makes it easier to update syntax to all our Dockerfiles at once as we learn best practices
5. adds a bash helper to try and deal with the extra complexity around `git subtree`
6. as a demo, pulls in `mandrill` and `sputnik`

#### How to review this PR

This PR adds a lot of extra commits because it pulls in the full history of other projects. The important files to look at are the two new files in the root: subtree.sh and README.md.

closes #1

#### Cons/Smells

1. extra complexity
2. difficult to reference github issues because the same commit message can end up in multiple repositories.
3. `git branch --all` is messier because of all the remotes
If every image has a remote aren't we still going to have _n_ projects for _n_ remotes?