Fix launch errors, improve Pods API, use hook abstraction

Sorry for the largeness of this PR, just took a while to get it all together. @mpuncel @petertseng 

This extracts the preparer's orchestration logic to a package that can be tested. It does not solve the self-hosting issue yet, but gets us most of the way there.
This is replaced by use of `p2-bin2pod`. 

```
$ godep go build ./bin/preparer
$ p2-bin2pod preparer
```