Add defaultUrlSchemeSuffix FBSession

The documentation on `initWithAppID:permissions:urlSchemeSuffix:tokenCacheStrategy:` in `FBSession` states that when `urlSchemeSuffix: nil` the information would be pulled from plist. But that was not the case. Only the appId was pulled from global default which falls back to plist. [see here](https://github.com/facebook/facebook-ios-sdk/blob/2a030e795aed343e7614e149a5ebdfa78b223a72/src/FBSession.h#L245)

This pull request lets you specify the default urlSchemeSuffix via "FacebookUrlSchemeSuffix" key from applications plist. And also set it programatically via global setter.

    [FBSession setDefaultUrlSchemeSuffix:@"suffix"];