Heat the sugar and water in a small saucepan over medium high heat. Stir to dissolve the sugar, and once the mixture starts to simmer, reduce the heat to low. Add the lime zest and juice, and allow the ingredients to simmer gently for 20-25 minutes, until the mixture has thickened and slightly reduced. The syrup may take on a slight golden hue, but do not let it darken too much - you do not want it to to caramelize.