Try to parse  access_token response.body as JSON

It looks like Facebook Graph API is now returning JSON response for the "access_token" end-point. This change updates the `parse_access_token` method to try to parse the response as JSON before doing it's own parsing. 

In the current version (1.10.1), the `OAuth#get_access_token_info` method returns something like:

```Ruby
{"{\"access_token\":\"CAAFXHuBquhABA0B8N7ATIXe8rMx9hyU6sGl6mEJg6nJeaJRIr6c60StNjaONm9Ks0ZCOSRt1SxS1F7LT4rI0D3wcoEYGD9n37WmQxWBsSsJMYWHjqERoLAodfaMnbkKkBIZAXZBBrZAJJpM5duM3QDZCoZAlVOfglGwgEE2LhrONANmeNQcpez\",\"machine_id\":\"C0RhVJb0EHTWTxloOa4P0Jvp\",\"expires_in\":5170749}"=>nil}