Current status of shortbread

## Summary  

* The server running as the CA can issue new certificates
* git integration is done. 
  * server writes its certs to a local git repo, which is synced with the remote after every local write. [shortbread-test](http://www.github.com/joshi4/shortbread-test) 
* The client can poll the server and add new certs to the ssh-agent. 
* The discovery-json api is fleshed out ( changes may occur depending on any server side changes to implement revoke) 
* command line tool is fleshed out as well
* successfully tested it using 2 `ubuntu` instances on ec-2 to log into a `coreOS` instance. 

#### Immediate Next Steps

* containerize the server, to get it running on kam. 
* blogpost ? 

#### Next Steps 
* ability to revoke certs
* write unit tests 
* synchronization across diff CA's 

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## Package level notes/questions 

### cmd/shortbreadctl

* do we want to provide a --server flag which can be useful in one-off cases when we want to override the settings of the environment variable specifying the url of the server to connect to ? The flag does nothing for now 
* Will change the `update` sub-command back to `useradd` before the next PR. 
* **Improvement:** since the root shortbreadctl command is not runnable, but I've provided a set of persistent flags to it (for ease of reuse in the sub-commands) would be less confusing if I were to override the default help template and roll out my own. 

### gitutil

* the libgit2 package provides the go bindings but they are too low-level. We want to provide an api that maps almost exactly to the interface provided by the git cli. I think it'll be a good standalone package to open source. So should I move it to a separate repo and submit a PR to coreos/gitutil ? 

### server 

* `url` and `path` hardcoded for now, will make them environment variables in the next PR. 
* ignore `Revoke` for now 
* in order to efficiently implement revoke might have to change the global map type to `map[fingerprint]map[nameOfPrivateKey]*ssh.Certificate`
will do, I had initially put it there but since, it wasn't used elsewhere I put it here.