Change the way how recurring jobs are being scheduled

* Scheduler now works only with local time - CRON format by default uses
local time.
* Scheduler now works based on LastExecution time instead of
NextExecution, fixes #158, #160.
Make scheduler classes internal
Switch scheduler back to use UTC time
Add hint about UTC time to the dashboard
Fix ScheduleInstantFactory class
Merge pull request #193 from HangfireIO/recurring-jobs-workout

Recurring jobs workout