Adding support for passport 0.2.x and some improvements

Hey @rkusa please check this PR, it adds `passport@0.2.x` support while still supporting older versions, I needed this for a project I am working on. I also added some other improvements like comments and small codestyle changes, added `transformAuthInfo` method and making sure `user` is assigned to `options.assignProperty`. 

No tests so no way to be sure there are no issues but I tested it with the `koa-passport-example` and it all work well, if I had time I would have added some tests.

BTW, great plugin!!!