Removed -D haxe3 compilation warnings
Fixed more Haxe 3 regressions (thanks to cambiata)

Commit tested with v3-00-rc1 branch. Fixes suggested by cambiata (https://github.com/cambiata)
Reference ticket : https://github.com/njuneau/Harfang/issues/6
Added version information and updated copyright headers
Updated Unit2 dependency
Updated documentation generation command
Updated Haxe libraries license informations
Fixed documentation, updated release information