Safe string support and remove camlp4 hard dependency

• Add support for -safe-string in OCaml 4.02 upwards.
  The main change is to rename blit_to_string to blit_to_bytes and
  change its type so that it writes to bytes rather than string
  (#74 by @yallop).
• Remove strong build-time dependency on camlp4 in the base library.
  The sexplib functions were only used in the interface, so replace them
  with manually written ones. This also enables compatibility with latest
  Core that has switched to ppx.
• Add multi-distro testing via Travis/Docker containers.