Add std::optional<> implementation.
Move optional into std14:: namespace.
Dumb down std14::optional, so it'll work on gcc 4.6.
Add more simulated C++11/C++14 features.
Add new value descriptions.
Refactor the OpenRTB structures.
Fix AppNexus "support".
Add automatic casting support.
Temporary: drop dependency on https://github.com/datacratic/soa/pull/22.