Implement define= for mp scenarios and eras

Implemented reloading of configs with define= attributes in both mp_create and mp_wait::join_game().
Added myself to about.cfg and updated changelog.

Conflicts:
	changelog
	src/gamestatus.cpp
	src/gamestatus.hpp
	src/multiplayer_wait.cpp
Add define functionality to modifications.

Added a new method, void create_engine::prepare_for_era_and_mods(),
which sets era and mod in game classification.
Prepare_for_[scenario, campaign] then set their respective defines and reload.

Conflicts:
	changelog
	src/gamestatus.cpp
	src/gamestatus.hpp
Fixed reloading for commandline mp
Implemented defines for largest mainline scenarios
Fixed scoping in multiplayer commandline load

As AI0867 pointed out, config object references were available after they had been made invalid by reloading
Merge pull request #216 from Rift-Walker/scenario_era_mod_defines_mergable

Add define functionality to scenarios, eras, and mods