Passport-oath2 broken check for 'trust proxy'
I'm using passport-facebook which in turn uses passport-oauth2.  It looks like there is a line of code explicitly for express buried down in it that causes loggin in to crash.

If you look in the file below the offending line is 19:
https://github.com/jaredhanson/passport-oauth2/blob/master/lib/utils.js 

```
exports.originalURL = function(req, options) {
  options = options || {};
  var app = req.app;
  //this is the bad stuff
  if (app && app.get && app.get('trust proxy')) {
    options.proxy = true;
  }
```

app.get('header nonsense here') is an express function and doesn't exist for koa.  Maybe add a mapping to koa to retrieve the header values according to what express returns?
