Merge pull request #3 from midoringo8823/filter-transformer

Adding filter transformer and tests.
Fix test for the changes on ChartDataObserable -> Observable, changing transformers to extend ChangeNotifier instead of Observable.
Merge pull request #17 from midoringo8823/filter-transformer

Fix test for the changes on ChartDataObserable -> Observable, changing t...
Transpose transformer and test
Change extending Observable to ChangeNotifier