Move randbytes() function into util.py

Also rewrite it to use random.getrandbits() instead of random.sample().
Move padPKCS7() function into util.py
Implement CBC-MAC for challenge49
Implement API server and client for first part of challenge49
Implement first part of challenge49
Implement API server and client for second part of challenge49
Implement second part of challenge49
Implement outline for challenge50
Generate collisions for challenge50
Generate collisions with only printable characters for challenge50
Implement Javascript version of challenge50 (for extra credit)