Add get_problemsets api and some other util functions

How to try it: (need register at http://gf2-roba.appspot.com/a/signup and log in at http://gf2-roba.appspot.com/a/login)

At first, add a new problem set (now this api is just for testing)
http://gf2-roba.appspot.com/a/add_problemset?problemset_name=test_problem_set
// change "test_problem_set" to another name

Try to get problem sets info:
http://gf2-roba.appspot.com/a/get_problemsets

Then assume you finish this problem set (need login, this api is also for testing now)
http://gf2-roba.appspot.com/a/try_problemset?problemset_name=test_problem_set

Try to get problem sets info again:
http://gf2-roba.appspot.com/a/get_problemsets

You can see the "has_completed" field has been changed