Fixe for access_token expiration problem.

[jdsingh]

fixes #17

Fix issue with extend() and default value of expires_at

Changed setExpiresAtFromTimeStamp() because facebook is sending seconds to expire in response rather than TimeStamp

and Facebook introduced never expiring tokens and 1 year age would be enough for those tokens.

[SammyK]

Thanks for the PR @jagdeepsinghmundi!

1. Make sure the tests still pass after making changes. This PR has failing tests in it.
2. Check out the fix I made on official PHP SDK that addresses this issue. The PR needs to accommodate both expires_at and expires return values.

[jdsingh]

Sorry I broke the test.

I was running phpunit on old branch and thought it passed.

And thanks for fix.

[SammyK]

No worries! If you just port over the fix and make sure the tests pass I'll merge it in. :)