TEST: Move to BOOST_REQUIRE test macros instead of assert, to be able…

… to perform tests in release mode (which disables assertions in favor of speed).

CONFIG: updated Hunter distribution to current release (nlohmann-json…

… and Boost version bump).

 * FEATURE: pre::retry_for (Function which retries a callback during …

…a given duration)

  * Support build on MSVC 2015 update 4

  * Include mapbox.variant and support of it for pre/json/[to, from]\_json and for Boost.Spirit.Qi/Karma.
  * `pre::variant::apply_visitor`, `pre::variant::get` unified for Boost.Variant / Mapbox.Variant to make the variant type easily switchable.
  * FEATURE: pre::variant::polymorphic_get implemented for mapbox::variant as well.

  * BUGFIX: Fix #5 thread datarace in mockup_serial_port.
  * BUGFIX: Mockup implementations of boost::asio::flush & boost::asio::get_bytes_available for code tested under mockup_serial_port.

  * [BREAKING] REFACTORING: This is a change in the api of to_hexstring(const string) into buffer_from_hexstring.

BUGFIX: Somehow the check for nothrow_move_constructible ends in an i…

…nfinite recursion on clang 3.9 with some of the types we have.

CONFIG: lib-cpp-pre 1.4.0.

CONFIG: Made unit compilation and optional (only interests pre develo…

…pers) and bumped version number.

to_hexstring improved for std::string buffers, pre::retry_for and msv…

…c fixes.

BUGFIX: Support build on MSVC as well.

CONFIG: v1.3.5 with utilities for variant handling, asio serial port …

…extensions and new karma binary generators for CRC and size.

BUGFIX: Add forgotten include.