added let binding syntax
added let binding syntax

Conflicts:
	diylisp/evaluator.py

codecop/diy-lang,joelchelliah/diy-lisp,rcanepa/diy-lisp,fatso83/diy-lisp,couchemar/diy-lisp,kvalle/diy-lang,kvalle/diy-lang,beni55/diy-lisp,rcanepa/diy-lisp,couchemar/diy-lisp,beni55/diy-lisp,kvalle/diy-lisp,joelchelliah/diy-lisp,kvalle/diy-lisp,codecop/diy-lang,fatso83/diy-lisp