CRM-13973-qa : online pay later scenario partial payment

This achieves the below flow:

 - User visits online event registration live mode, and selects 'pay later option' and successfully completes registration. e.g. he records $50.
 - User visits Edit registration page of the above registration, 'checks' record payment checkbox, and records intended amount e.g $25. This also will change the participant status to 'Paritially Paid' automatically. (strictly for contribution).
 - Then the additional payment workflow will be same as currently handled.