add _transfer_cookie method (webauth)

[hexiro]

Added a method called, _transfer_cookie which sets a cookie for the three main steam domains.
This method removes redundancy of repeating:
for domain in ['store.steampowered.com', 'help.steampowered.com', 'steamcommunity.com']:
by handling that with this method. Let me know what you think of this idea!

[rossengeorgiev]

This servers no purpose besides adding another call to the stack for the sake of not repeating one thing twice. It taking DRY to an extreme. I think I must more reasonable thing would be to have the domains in variable.