Simplify the password format checking.

1. The minimum length is set to 6. Because a lot of people's debit cards
use 6 length passwrod. They may wanna use the same with Peatio.
2. Remove the checking for the format of a password. We just give suggestion.
Merge pull request #115 from peatio/simplify_the_password_format_check

Simplify the password format checking.