Complete POP3 functionality, moved locker, imminent parser rewrite

Complete POP3 functionality.
Added the following POP3 commands: DELE, NOOP, QUIT, RETR, RSET.
Better command implementation.
Added pop3/sentences package to parse the config values (from package config) and return elaborate strings.
Added the following POP3 settings: "start_greeting" and "end_greeting".
Moved locker, but probably won't use it in SMTP (this will lead to a change in the POP3 code: will have to store the state of the mailbox at the start of the session).
Started commenting the code for godoc.
General POP3 code cleanup.
mailboxes package.
pop3/message package.