Configurable behaviour for cookie defaults and overrides

[elengine]

I offer one solution on the issue #2.

[georf]

Please merge this and create a new gem version!

[pschinis]

I haven't merged this because I think it goes beyond the scope of this gem. The gem is really just intended to patch over the behavior of older rails apps so they can continue working as they were before in newer versions of Chrome. Given the direction browsers are going it's not really meant to be a permanent solution and most of these apps will need to find ways to move away from the use of third party cookies in the near future. It already doesn't work in Safari unless the user changes their settings and there's talk of Chrome eventually disallowing these kinds of cookies in a similar way as well.