Updated such that all tests now pass.

Problems arose out of the fact that 
martian was scanning the __builtin__ namespace
for context directives.

src/martian/context.py

@@ -1,5 +1,6 @@
 from martian.directive import UnknownError
 from martian.util import scan_for_classes
+import __builtin__
 
 class GetDefaultComponentFactory(object):
     def __init__(self, iface, component_name, directive_name):

src/martian/directive.py

@@ -132,8 +132,8 @@ def _default(mro, get_default):
     """
     error = None
     for base in mro:
-        if base is object:
-            continue
+        if base.__module__ is '__builtin__' or base is object:
+            break
         module_of_base = scan.resolve(base.__module__)
         try:
             if util.is_baseclass(base):
@@ -191,8 +191,8 @@ def get(self, directive, component, get_default):
         mro = inspect.getmro(component)
         # look up module-level directive for this class or its bases
         for base in mro:
-            if base is object:
-                continue
+            if base.__module__ is '__builtin__' or base is object:
+                break
             module_of_base = scan.resolve(base.__module__)
             result = directive.store.get(directive, module_of_base,
                                          _USE_DEFAULT)