Consider OpenFL .hxp files as haxe files

see http://www.openfl.org/documentation/projects/project-files/hxp-format/
FileHelper.IsHaxeFile() -> IsHaxeExtension()
Merge branch 'development' into openfl_hxp

Conflicts:
	External/Plugins/ASCompletion/Model/FileModel.cs
Merge pull request #421 from Gama11/openfl_hxp

Consider OpenFL .hxp files as haxe files