Add 'args' parameter to pseudo commands.
Implement more accurate parameter formats for each function.
Add feature to set the fake maildrops.
Fix do_retr() to return user-defined mail data
Substitute complicated logics with simple fake data.
Add pseudo data for testing.
Simplify the implementation of 'POP3Server' class
Define how the mock server responses to requests.
Fix several bugs

* Cancel the alarm after delivering a mail successfully.
* Don't remove tmp file immediately.
* Execute sleep(2) even if os.stat() didn't raise any exception.
Purge the unnecessary import statement.
Improve the format of get_uniqueid() method
Add test cases for 'Maildir' class.
Improve the test case for readability
Set up maildir-format directory for testing.
Check if the commands are issued as planned.
Check the state of mailbox after 'fetchmail()'
Add more test patterns.