added ability to login to edx before transactions

you can now optionally specify a url in the config file as a list. e.g.:  - ['https://foo', true]
the boolean element tells the profiler whether or not it should login to the app before running the transaction.

note: if you specify true to login, you must also configure the fields for "login_user", and "login_password".

/cc @benpatterson @jzoldak 
I think it's ok to be in code since it should never change, and specifying a different login page would most likely break _login function.