Use NSDate instead of GCD time in RACScheduler
Add RACQueueScheduler method for calculating walltime
Refactor schedulers and unit tests to use NSDate
Merge branch '2.0-development' into date-based-scheduling
Fix argument indentation
Fix method name in RACImmediateScheduler
Merge pull request #664 from ReactiveCocoa/date-based-scheduling

Use NSDates for scheduling, instead of dispatch_time_t