Add tpl property and __repr__ method to SExp
Add tpl property and __repr__ method to SExp

SExp.tpl returns a tuple representing the SExp; children are left as SExps,
 not converted to tuples.
So for instance, SExp.parse('(a (b c))').tpl will equal
 ('a', SExp.parse('(b c)'))
There is currently no method that would turn that SExp into
 ('a', ('b', 'c'))

ec429/sexpy