Allow setting the result of a task completion source from an existing task.

Created by: richardjrossiii

Frequently, I have to write code like this:

BFTaskCompletionSource *source = [BFTaskCompletionSource taskCompletionSource];
 [.... continueWithBlock:^id(BFTask *task) {
     if (task.faulted) {
         NSError *error = task.error;
         if (error) {
             [source trySetError:error];
        } else {
             [source trySetException:task.exception];
         }
     } else if (task.cancelled) {
         [source trySetCancelled];
     } else {
         [source trySetResult:task.result];
     }

     return task;
 }];

It would be nice if there was an API similar to [source trySetTask:task] instead of having this ugly block of code.