Add salt support to blog.utils.hash_repeat.
blog.model.User use str(register_time) as salt_pre now.
Add a get_password_digest method to User.

This method will use self.register_time as salt_pre to calculate the
hash value of a password.
__init__ use get_password_digest to get password's hash value now.
Add a function to blog.utils.

The function is remove_microsecond. It will return a new datetime object
same as its arg but microsecond is 0.
Add a empty in the docstring of blog.utils.hash_repeat.
blog.model.User.__init__ will leave out the time's microsecond.

When we use model.commit(), we will find that our user object's ervery
datetime key's microsecond is left out. So we will leave out the
microsecond of the register_time and last_login_time to keep step with
the database.
Supplement the instructions of the salt used by User.__init__.
Rename blog.model.User.get_password_digest as get_password_hash.