[sugar] Activity API changes

[Marco Pesenti Gritti]

Another couple of changes.

1 In activity.info there is now a class attribute for activities written
in python. Instead of:

exec = sugar-activity-factory webactivity.WebActivity

You should now have something like:

class = webactivity.WebActivity

2 To get the bundle path you now should do:

from sugar.activity import activity
bundle_path = activity.get_bundle_path()

I don't have more changes planned on the short time, so, unless problems
arise, this should be all.

Marco