Add a root_app property to cli.Application. Closes #140

This adds a property to `cli.Applicatino callled root_app which returns a
reference to the "root" Application object in a tree of Application objects.

Conflicts:
	plumbum/cli/application.py

plumbum/cli/application.py

@@ -137,6 +137,9 @@ def __init__(self, executable):
                     self._switches_by_name[name] = swinfo
                     self._switches_by_func[swinfo.func] = swinfo
 
+    @property
+    def root_app(self):
+        return self.parent.root_app if self.parent else self
 
     @classmethod
     def unbind_switches(cls, *switch_names):