Store Facebook expiry token as returned by the service

Facebook returns 2 kinds of tokens depending on what endpoint
we are talking to:

1. Short-lived tokens: Tokens used in client-server OAuth,
   have a short expiry time of 2 hours

2. Long-lived tokens: Tokens used in server-server OAuth,
   have a longer expiry time of 60 days

Previously we were storing a hard-coded value of 2 hrs for it
as there was some confusion regarding these values. But now
we'll store whatever value the service returns.

Also, Facebook doesn't follow the OAuth2 RFC and returns a
differently named expiry token - expires, so we'll use this
special name in the O2Facebook class.