Impersonation from config

I'd like to be able to _always_ impersonate because I basically only use gemfury as an organization. 

I'm submitting this code as a possible solution. To accomplish changing impersonation, you change your `~/.netrc` file so that the `login` field is the organization's username, rather than your email address.

```
machine www.gemfury.com
  login myorganization
  password abCdEFgHiJkLMnopqRSt
```

If this seems like too much of a hack, I'm happy to consider having this in the "legacy" `~/.gem/gemfury` file, but I didn't want to add a dependency on a config file you were deprecating.