support for LuaJIT-2.1 + LuaJIT built with lua52compat

Signed-off-by: Vadim A. Misbakh-Soloviov <mva@mva.name>
Progress handling (fixes #14). To be tested.

Signed-off-by: Vadim A. Misbakh-Soloviov <mva@mva.name>