Support webservice repository

This patch is a refactoring of the accounts manager for webservices to
enable services to be installed in both /usr/share/sugar/extensions
and ~/.sugar/extensions. The complexity is due to an issue with a
collision of namespace; hence a repository of loaded modules is
maintained to help disambiguate services installed in the two places.

(Now using pkgutil instead of local namespace and import_module helper method for forward compatibility to Python 3.0)