ZArith v1.8

• Request #20: infix comparison operators for Q and Z [Max Mouratov]
• Request #39: gdc(x,0) = gcd(0,x) = x [Vincent Laporte]
• Request #41: support for upcoming OCaml 4.08 [Daniel Hillerström]
• Issue #17: add package zarith.top with REPL printer [Christophe Troestler]
• Issue #22: wrong stack marking directive in caml_z_x86_64_mingw64.S [Bernhard Schommer]
• Issue #24: generate and install .cmti files for easy access to documentation
• Issue #25: false alarm in tests/zq.ml owing to unreliable printing of FP values
• Request #28: better handling of absolute paths in "configure"