Add empty shell of POP3 server
More basic POP3 implementation

Filestore/Datastore now supports a Size() method.
Started impl DELE
Started impl DELE
Most of RFC1939 implmented

Most of the stuff in RFC1939 is implemented, and I was able to download
messages with Thunderbird.  We do not yet purge deleted messages, and
probably need to implement some more auth modes.
Impl deletes, update SMTP session tracing
POP3 is working

Added pop3.domain config option (for APOP greeting)
Implemented CAPA command
Implemented APOP "encrypted" authorization
Updated all sample config files to include [pop3] section
Closes #8
Merge pull request #13 from jhillyerd/pop3

Merge POP3 changes into master