added a premake5.lua file to allow using the latest premake-dev versions

Hi,

This pull request just adds a Premake5 script (which fixes a warning on the deprecated "Optimize" flag) which in turn allows the creation of Visual Studio 2013 solutions.

As a side note : the libs (TinyXml and wxWidgets) that are in the decoda repository were build with the Visual Studio 2013 SDK, not the vs2012 one, like it's written in the readme file : If you clone decoda on a fresh folder, then run "premake4.exe vs2012" and try to compile with Visual Studio 2012, you'll get a _MSC_VER mismatch. With this pull request, if you run "premake5.exe vs2013" and try to compile with Visual Studio 2013, no more mismatch.