Add POP3Agent class
Implement login() method (APOP supported)
Add 'Maildir' class to support maildir(5) format
Add 'Maildir.get_uniqueid()' method.

The current format is '{unixtime_in_ms}.{pid}.{hostname}';
We might need to put more complicated token in the format,
if it is not enough to guarantee the uniqueness.
Implement Maildir.deliver() method accoding to maildir(5)
Implement the mail retrieval functionality.
Add new class to support POP3 over SSL.
Add the procedure to generate a SSL context.

* The implementation is largely based on ssl.create_default_context()
  (with some security enhancement).
* I think TLSv1.0 should be disabled as well, but the problem is that
  TLSv1.1/1.2 is suported natively only in Python 3.4 or later. Should
  we kill Python 3.3 support for the sake of better security?
Add __init__() method to POP3AgentSSL.
Add a simple function to parse config files.
Add a method to process password.
Add main() to pop3agent.py
Add base class to mock the server functionality for testing.
Implement the pseudo POP3 server.
Add a simple test runner.
Add some functionality tests.