add username validation check to on register

I've tried to register a user with "." in the username and found this prevents me from successfully pushing an app to the builder.

Further research yielded these lines:
```
docker push 10.88.103.88:5000/n.friedman/example-ruby-sinatra
2014/02/27 11:29:38 Invalid namespace name (n.friedman), only [a-z0-9_] are allowed, size between 4 and 30
```
I've run this when attached to the builder container using `lxc-attach`

Another option to solve this would be to sanitize the images name, but this could result in similar names for different users, which is probably not good.