check if refreshRate is set on init, if not use default

Hi,

I found your plugin and it's great. Love it.

But I had an error when initializing the plugin.
After calling `sagPanZoom('#mySVG')` the code crashed at `SvgUtils.refreshRate = options.refreshRate;` because `options.refreshRate` was`undefined`.

I added an workaround in my code to avoid that error.

Great work,
Artur