Code review on HolidayRequest

A much cleaner/simpler design than the previous one.

Extracting the email server is nicely done, the HolidayRequest class is clean and tidy.

It can be further simplified by:
1. adding a manager to the HolidayRequest (that is a missing feature: mail should be sent to the manager when a mail is created)
2. dropping the address book (EMployee, Manager can have their own email, HR email is unique, can be in application configuration)

Also the abstractions around emails and sending emails can be improved:
1. create an Email class (which is everything an email is, like from, to, subject and body and send() operation
2. let it talk to the EmailServer 
This way the HolidayRequest does not know about email server concept..