fixed a little bug and add an improvement

Improvement
- check whether a username is already exist in database.users.keys().
  So the users don't have to fill all boxes and find the fact that they choose a username that already exist

Bug
- just a little mistaka. Using the original main.py, we'll got an error.
  The error araised: SyntaxError : invalid syntax because the original main.py will write a wrong syntax to
  Account.py.

Another thing
- after I 'sign up', I can't login immediately. I need to re-run again main.py (I use IDLE, btw).
  I think it's related with read/write thing in main.py code.