NMS-6682: Initial revision of Pop3Monitor

http://issues.opennms.org/browse/NMS-6682
We could add something similar like in the ImapMonitor:

    Connected to mail.mydomain.de.
    Escape character is '^]'.
    +OK Dovecot ready.<1>
    quit<2>
    +OK Logging out<3>
    Connection closed by foreign host.

    <1> Test POP3 server banner, it has to start with `* OK` to be up
    <2> Sending a `quit`
    <3> Test server responds with, it has to start with `* BYE` to be up