[#386] set default value to `1.0` for `setOpacity()` if the given value is out of range (e.g. Nan)
[#386] fixed regression with the collision layer not being set with a renderer  and not being added to the game world when visible
don't draw fully transparent Tile Layers

this both allows to add hidden layer to the game (and change the
visibility status later) AND to still "silently" managing the collision
layer
Fixed documentation for `me.sys.stopOnAudioError`

I wonder if we should still keep this one actually
Removed last traces of `me.debug` in the engine, and restored the collision layer visibility flag/checkbox in the debugPanel

me.debug and child flags/settings are now "managed" only by the debug
panel !
[#457] Added path variables to grunt file for performance and organization (@DuckyCrayfish)
[#457] various documentation fixes (DuckyCrayfish)
Merge remote-tracking branch 'origin/1.0.0-dev' into 1.1.0-dev