Melt INSTALL.txt, THANKS and README.txt to new README.rst

This is to fix #760. For the Windows portable version, insert of AUTHORS and LICENSE to the portable is still missing. Also the line endings have to be checked. @emanuelschuetze Can youy have a look at it? Be care of the write_readme function in the prepare_portable script because it manipulates the readme and adds package and license section. Please check whether you want to change something here or not.