add enable enter key to login

[ivanyschen]

I created another PR to adopt the functionality of logging in with enter key to the new interface and closed #42
@HarowitzBlack , apparently, there is a conflict between bind and overrideredirect (1) methods. I disabled the overrideredirect because I think it is okay if users just wanna close the window if a login failure happens. Tell me what you think.

(1) https://stackoverflow.com/questions/31664537/unable-to-bind-keypress-in-tkinter

[iamnottheway]

@ujhuyz0110 yeah sure! Maybe It's good to disable it. :)