User profile upgrade

I replaced hard-coded list of fields with configurable one.

I preserved the old fields as a default ones for full backward capability.

When `profileFields` is specified I'm extending default fields with it. I thought it's a better idea then overwriting.

I also improved `VK.com` response parsing and moved it to a separate module.

I think it'll be a good idea to remove `username` from parsed profile, because it's against `passport` [User profile notation](http://passportjs.org/guide/profile/) and [Contact schema](http://portablecontacts.net/draft-spec.html#schema) it's based on.
But I preserved this field for backward capability.

I also extended readme.md with `Profile Fields` sections.

I thought to write tests for my code as well, but I was unable to run your current tests.