Better documentation pages.
Minor cleanup stuff.
Output language "cvc3" (as opposed to "cvc" or "cvc4") produces output for CVC3:

1. no decimals used for rational literals
2. queries/check-sats wrapped with PUSH/POP
Fatal error if --unconstrained-simp and --produce-models used together (before it would just override the user and turn off models).
Fix compiler warnings in arith and ITE utilities.
Add LETP (parallel let) to CVC4 native language.

Change CVC4 printer to use LETP by default.  CVC3 printer has no LETP.