Proxy headers now also take a function

Use it like this:

```
'Authorization': function(req) {
  accessToken = (req.user && req.user.access_token) ? req.user.access_token : 'api_token'
  return 'Bearer '.concat(accessToken)
}
```
Merge pull request #15 from ehtb/patch-1

Proxy headers now also take a function