v0.2

CHANGES:

• Add pretty-printer
• Randomize pop action on the fuzzer
• Add tests on Rke and Rke.Weighted (with alcotest)
• Fix bug retrieved by ocaml-git (see 453633b)
• Add rev_iter function
• Fix bug on Rke.N.keep_exn (see 3951501)
• Add Travis CI support