Feature: added support for collecting popup (confirm/alert/prompt) messages
and providing default value for different Javascript popups.

1. All the different popup messages collected in popup_messages list.
2. You can pass default value to use for confirm/alert/prompt dialogs, without
the need to use the with statement (in case you just want the popup to go away).
3. Little refactoring to GhostWebPage: I'm passing the Ghost object to it,
so we won't need to use global variables. This refacotring can later be applied
to _alert and similar variables.