function call_to_read_a_file(){
readFile(fs,'/path/to/file.txt',function(res){
    alert(res);
});
}
function readFile(fs, file_path, success_fn){
    fs.root.getFile(file_path, {}, function(fileEntry) {
        fileEntry.file(function(file) {
            var reader = new FileReader();
            reader.onloadend = function(e) {
                success_fn(this.result);
            }
            reader.readAsText(file);
        }, errorHandler);
    }, errorHandler);
}