Convert popup message to unicode rather than str.
Make sure the result value in GhostWebPage.javaScriptPrompt is always string (otherwise error occurs).
Only set Ghost._prompt_expected/_confirm_expected if default_popup_response is not None.
Do not clear Ghost._confirm_expected/_prompt_expected (it gets cleared anyway by the with statement).
Merge branch 'fix_popup_collection' of git://github.com/EverythingMe/Ghost.py into EverythingMe-fix_popup_collection

Conflicts:
	ghost/ghost.py