Implement random assignment

[FedoseevAlex]

Sometimes people can't decide a date for duty. Or they don't really care.
For this kind of users random assignment should be implemented.
Command form proposed as: /assign random or any prefix of the word random. (so /assign r should work too).
This "randomness" should be limited for two weeks ahead from command execution moment.
Let's consider next example:
Today is Tuesday and user0 want to assign random
Schedule:

week	weekday	user
1	Mon	user1
1	Tue	user2
1	Wed	free
1	Thu	free
1	Fri	user3
2	Mon	user4
2	Tue	free
2	Wed	user1
2	Thu	user2
2	Fri	user0

Free days on first week should be used first. After that free days on next week should be used.