Add undefined check for "options.data" in addHeatMapLayer: function (map, options)

[allanes87]

Hi,
Firtsly, thks for this API!
I found a little issue to improve:
In "jmelosegui.googlemap.js", function: "addHeatMapLayer: function (map, options)", Is possible add a undefined check:

var data = [];
            **if (options.data !== undefined)**
            {
                for (var j = 0; j < options.data.length; j++) {
                    data.push(new google.maps.LatLng(options.data[j].lat, options.data[j].lng));
                }
            }

Without this little improve I have this error:

Uncaught TypeError: Cannot read property 'length' of undefined.

[jmelosegui]

You PR is wellcome!!!