better behavior for special cases such as builtins (signature throws error);
isvalid now calls func when is a builtin; _keygen operates in safe-mode;
added tests for the above


git-svn-id: svn+ssh://svn.mystic.cacr.caltech.edu/pathos/klepto@564 8bfda07e-5b16-0410-ab1d-fd04ec2748df
Merge branch 'master' of https://github.com/uqfoundation/klepto