Remove explicit headers for live urls

I was never fully convinced the User-Agent was needed here, but the
Keep-Alive is now automatically handled/set with Requests. Thus, these lines
are no longer necessary.

Issue #61
Move start_addon() to game_xbmc.py
Move addon_profile check to game_globals.py
Overhaul login/auth system

GUI and logic are now finally separated.

check_login() has been split into two different functions: login_gamepass()
and login_rewind(). These functions are responsible for all aspects of the
login process (already logged in, sans-login, blackout, actual login,
and login verification).

gamepass_login() is renamed to login_to_account(), and now no longer calls
check_for_subscription() to determine success or failure. That is the
responsibility of login_gamepass() and login_rewind().

These changes /might/ break "login" for those in sans-login regions.
This needs to be tested, but the fix is easy. My hope is that sans-login
regions still have a "subscription" returned. If so, then the sans-login
setting can be removed altogether.