email: add auto fetch attachments for POP3 accounts

This change adds an option to allow auto fetch the complete POP3 message.

Change-Id: Id92a46a8f84a575160f04003f72dfe513441d356
Signed-off-by: Jorge Ruesga <jorge@ruesga.com>
Merge "email: add auto fetch attachments for POP3 accounts" into kitkat