Now defaults parent to tk master
Refactored LoginDialog to use gui/dialogs/login

Removed ChangePasswordDialog
Refactored online.py into tutorlib/online

Only very minor changes.  Broke up a few parts just to be consistent
with how the rest of MPT is structured.

Minor changes to SessionManager API (no longer need to leak the SEVER
url from the file).
Refactored WebAPI & SessionManager

SessionManager will now throw exceptions rather than printing errors to
stderr.  WebAPI is aware of these exceptions and handles them.

Cleaned up the internal WebAPI interface quite significantly.