Is it possible to aggregate credit card statement information legally? If so, what are the steps, and what things do I need to keep in mind?

I am looking to build a website which, among other functions, aggregates credit card transaction information (namely the amount of money somebody has spent on each of their credit cards over the last month). I haven't decided how (technically) I will do so, but the basic idea would be the user would provide their banking credentials, and then my site would go and scrape that cards site for the statement information. In this manner if the user has 2 credit cards, with two companies, they can come to my site and see spending information for both cards.

Mint.com does just this (though I just care about amount spent and not all the individual transactions).

However, I imagine Mint.com (Intuit) has an extensive legal team, that have written up extensive legal agreements with each of the credit card companies. I am one man, that maybe can afford a single lawyer. That said is there a legal way for me to do what I'd like to do without the need for massive litigation? What steps do I need to follow, and what things do I need to keep in mind?