Allow index specs to be composed from raw strings

This allows an index spec to reference arbitrary keys of a DictField:

    class MyDoc(Document):
        frobs = DictField()
        meta = {
            "indexes": ["frobs.fmep", "frobs.gorp"],
        }

mongoengine/base/document.py

@@ -666,8 +666,14 @@ class BaseDocument(object):
                 fields = []
             else:
                 fields = cls._lookup_field(parts)
-                parts = [field if field == '_id' else field.db_field
-                         for field in fields]
+                parts = []
+                for field in fields:
+                    try:
+                        if field != "_id":
+                            field = field.db_field
+                    except AttributeError:
+                        pass
+                    parts.append(field)
                 key = '.'.join(parts)
             index_list.append((key, direction))