get_access_token broken

[gmrodrigues]

Hi,
First thing: thank you for this lib. Really good job.

Now the issue: I think something changed on facebook's service that broke the get_access_token funcionality. It's simple: they are now sending json in response.

Im thinking about sending you a fix for this, but i need you to confirm the issue first. Maybe its simpler for you just to fix it, or maybe i'm wrong.

Thank you =D

[SergioDzul]

The response of facebook now come in javascript json like u'{access_token:"5dfsdfsd.."}' i send the pull request to resolve this, but i don't sure if @gmrodrigues accept, because is a simple fix.

c50302f