Fix#1 display visited page count

[haichuan0424]

I examined the code and found out that when the user tries to login, the visit record does not include the "page" key, so the visits collection has null values for page. When the visits.aggregate function in the mongo.js file tries to create _id based on page, there will be null value, so the aggregate does not return any data.

I fixed the exports.login function to only save to visits collection if the user exists and add a page key/value pair before saving to database. Please let me know if any issues or further need.

[Tubaleviao]

Nice, this is a good beginning. But this don't correct the error completely, we are still not able to see the user's visits.
Nice job!