Prevent unintended access to sensitive fields (passwords, private keys) (#876)

Make sure that fields specified in filter, sortBy, etc. are model fields
and may be accessed. This is fixes a potential security issue.

The filter() function allowed guessing the content of password hashes
one character at a time.

The sort() function allowed the user to call an arbitrary method of an
arbitrary model attribute, for example sortBy=id&sortDir=distinct would
produce an unexpected error.

lemur/exceptions.py

@@ -29,7 +29,7 @@ class AttrNotFound(LemurException):
         self.field = field
 
     def __str__(self):
-        return repr("The field '{0}' is not sortable".format(self.field))
+        return repr("The field '{0}' is not sortable or filterable".format(self.field))
 
 
 class InvalidConfiguration(Exception):