auth: provide a hash implementation parameter to the token function
db: added password_tokens collection

This collection will be used to store tokens for password resetting.

Related to #200.
auth: added function to create password tokens

Related to #200.
auth: added method on user to start password reset process

This method will create a token and send it by email to the user. The
next step is the validation of the token. I will add the ResetPassword
method that will receive the token and actually reset the password,
mailing the new password to the user.

Related to #200.