Add set bin item expiry time functionality

[ybchen97]

• Edit clear command description in user guide

• 178, #179, #180

[chaitanyabaranwal]

One bug with the functionality is this:

1. Set binitemexpiry to 60 seconds, delete a person.
2. Change binitemexpiry to 30 days.
3. Even though the bin expiry is internally changed to 30 days, on entering the command listbin, the expiry date for the contact still shows the date which delayed by 60 seconds.