Now we can set different divs where multiple chart for any single page

If we want to implement multiple chart in one page then it would be better to set just the **div_id** for each chart. So we don't need to define that in onstructor everytime
Update GoogleCharts.php
Calendar chart package added

By adding **calendar** package now you can create [Google Calendar Chart](https://developers.google.com/chart/interactive/docs/gallery/calendar#SimpleExample)
Merge pull request #18 from previewict/master

Multiple chart display on same page by assigning $this->div. Also now it supports Google Calendar Chart