v1.2.0

CHANGES:

• Add a signature for the populate function, and add basic implementations
  for all drivers (GPR#28).
• Add stream for result extraction (GPR#22 Nathan Rebours).
• Use the postgres driver when postgres is specified as the URL scheme
  (GPR#25 James Owen).
• Documentation improvements (GPR#20 Nathan Rebours, etc.).
• Reimplemented partly ineffective prepare-cache for PostgreSQL.
• Backwards incompatible changes to the driver API.
• Backwards incompatible changes to modules marked internal but exposed due
  to being split across packages.
• Fix forward compatibility past OCaml 4.08 as announced by deprecations.