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 
Not a must, but I wonder if we'd want to also reference the login page in the config file?