Handle scenario when username != repo owner

Previously, the Bitbucket service hook made the assumption that
the username and repo owner were always the same. This commit
adds a repo owner field to handle situations when this assumption
is wrong. If the repo owner field is missing, the hook should
fall back to using the username. Users that verified their
accounts using the previous version should not be affected.
As this snippet occurs twice, I would suggest extracting it to a private method that may work like so:

```
def repo_owner(config)
  if nil_or_empty?(config[:repo_owner])
    config[:username]
  else
    config[:repo_owner]
  end
end
```