Sets translate, and other short fixes

- $ is a simple symbol is smt2.
- ever found yourself counting in kind.h? no longer.
- expose parser "logic is set" state for smt/smt2 (any better way?)
- a more helpful assertion message in smt_engine
option to hide stats which are zero (off by default), also some aliases
-{d,t} help => --show-{debug,trace}-tags
sets: fix equality propagation
rm warning from helloworld example
Merge pull request #28 from kbansal/sets

Sets translator, bug fixes