Changing of login interface

[hanshsieh]

Hi, I did some changes related to the login interface, here's a list of changes:

• Change the login interface: Not login at the constructor. Instead, a user should invoke different login method to choose the way of login.
• Allow a user to define the system name and OS type.
• Don't store the certificate at the file. Let the users do it by their own selves if needed. (they may want to store it somewhere else, or they may want to use multiple accounts at a time, etc.) I'm welcome for a discussion. :)

[ruyaoyao]

Hi! Thanks for your effort. In fact, I didn't think it through while designing client interface, I am glad it still works nowadays. I don't see any problem from your PR, so I will just merge it.

Please let me know if you have other thoughts.