Modifications to follow up the new load API

[satoshieguchi]

I made some modifications to follow up the new load API of Google Charts.
In this process, I fully refactored the ApiLoader class to bring a much higher flexibility required to support future API updates which I can imagine at this time.
The default API version was switched from 1.1 to "current".
When you would like to customize the behavior of the loader, all you have to do is

1. creating a subclass of ApiLoader,
2. overriding
   • callNativeLoad() method (for a future API change),
   • onInjectionSuccess() method (in case that you prefer the version 44 or earlier APIs together with either Geochart or Map Chart),
   • onLoad() method,
3. to give the instance to ChartLoader.setApiLoader() method as its argument.

Note that limited functionalities of this library were tested, and any comments are welcomed.