add pie animate

Added an animate to pie graph using value animator issue #51. supports listeners and interpolators. I created an interface that only PieGraph implements for now. Line graph would have some non obvious behavior for new data that's different, and Bar Graph would probably want to implement stacked "shadow bars" for the old values. If this interface is acceptable I'll start on those.

It's only for api 12+ so I added checks, and it Logs an error message and does nothing if it's below that, since the min sdk for the library is 8.

Updated gradle to lastest version. Fixed issue #47 by changing totalvalue from an int to a float.