Create premake install action

[hroncok]

For now, there is no install action.

It would be great to have one, according to the docs, it is supposed to be done as follows:

newaction {
   trigger     = "install",
   description = "Install the software",
   execute = function ()
      -- copy files, etc. here
   end
}

We could just copy the files around form here, but it would be great to support both different PREFIX and DESTDIR.