omniauth-foursquare isn't compliant with the auth hash schema

[romanbsd]

Based upon the omniauth wiki -
https://github.com/intridea/omniauth/wiki/Auth-Hash-Schema

The image should be "A URL representing a profile image of the authenticating user. Where possible, should be specified to a square, roughly 50x50 pixel image.", however in case on omniauth-foursquare it's a hash.

In my projects I do something like this in order to normalize it (50x50 was proven to be to small for me):

image =  [image['prefix'], '100x100', image['suffix']].join

I think that it's a sensible approach to return a URL for info.image, and put this has in the 'extra' hash (if it's not there already).