Throw error when flot.time plugin is missing.

Fixes issues 709 on google code.

jquery.flot.js

@@ -1222,6 +1222,9 @@
                     return v.toFixed(axis.tickDecimals);
                 };
             }
+	    else if (opts.mode === "time" && typeof axis.tickGenerator === "undefined") {
+                throw new Error("Time mode requires the flot.time plugin.");
+	    }
 
             if ($.isFunction(opts.tickFormatter))
                 axis.tickFormatter = function (v, axis) { return "" + opts.tickFormatter(v, axis); };