update mailsms to recognized already fetched emails

[ippbx]

When using pop3 for email to SMS pop3 email is not deleted from mailbox, which causes message to be resent every minute when it checks the pop3 mailbox and rereads the message again.

[antonraharja]

this might be mail server's issue. but even if it is playSMS need to have a way to identify an email whether its new or already fetched and processed.

i'm scheduling this to later milestone.

[sbrawner]

I believe for IMAP it is possible to use message uid, however unknown if possible to do this with POP (unread maybe)... perhaps a hash function.