Fix some minor things.

-   Add a link to bcrypt-ruby.
-   Switch order of operands.
-   Change "way to do" to "way to go".
Use "user" instead of "account".
Add inline comment for notifying users.
Beef up intro with real-world examples.
Add rationale for removing old data.
Use "user_id" instead of "account_id".
Add a short conclusion.