issue #106: _unpickle_context() did not allow nameless contexts.

These are generated by any child calling .context_by_id() without
previously knowing the context's name, such as Contexts set in
ExternalContext.master and ExternalContext.parent.

mitogen/core.py

@@ -957,8 +957,10 @@ class Context(object):
 
 def _unpickle_context(router, context_id, name):
     if not (isinstance(router, Router) and
-            isinstance(context_id, (int, long)) and context_id >= 0 and
-            isinstance(name, basestring) and len(name) < 100):
+            isinstance(context_id, (int, long)) and context_id >= 0 and (
+                (name is None) or
+                (isinstance(name, basestring) and len(name) < 100))
+            ):
         raise TypeError('cannot unpickle Context: bad input')
     return router.context_class(router, context_id, name)