Add convenience method for delayed schedule on RACScheduler

Method `-after:schedule:` takes dispatch_time_t argument that is in nanoseconds.
New method `-delay:schedule:` takes NSTimeInterval as argument (in seconds) and
calls `-after:schedule:` with calculated dispatch time. Method is implemented
in suberclass and is not abstract.
Rename `-delay:schedule:` -> `-afterDelay:schedule:`
Merge pull request #386 from iMartinKiss/feature/delay-scheduler

Add convenience method for delayed schedule on RACScheduler