Add single_instance field to activity bundle

This field can be used by activities that need to be restricted to a
single open instance. Usage is:

single_instance = yes

jarabe/model/shell.py has been modified look for this field in a
separate patch to Sugar.

Together these two patches constitute the implementation of Sugar
feature Launch Limits [1]

[1] http://wiki.sugarlabs.org/go/Features/Launch_Limits