Made the util::Dir append function const safe
Added const specifier to asset_dir argument of audio_manager's
load_resources method
Implemented JobManager to load gamedata resources

The game loads straight away without waiting to load the gamedata and
once the data is loaded continues to behave as it has been doing before
this commit.
Fixed the intialization reorder compiler warning
Merge branch 'master' into gamedata-job

Conflicts:
	copying.md
	cpp/game_main.h
Made the on_gamedata_loaded function accept reference to gamedata
Made GameMain a friend of Engine

This can be removed once all the components have been properly moved
from GameMain.
Showing a text saying that the gamedata is loading
Merge pull request #137 from jprashanth/gamedata-job

Fixes #70