Enable the angular to use own scope per pie chart instance

The problem: using two pie charts on one controller is disabled, because both uses the parent scope of the "PieChart" controller. So i added the `scope` property to the [`Directive Definition Object`](http://code.angularjs.org/1.0.8/docs/guide/directive) https://github.com/mrzmyr/easy-pie-chart/commit/680e4eb456f042c6dfd72a8811d73b814be6ae1c#diff-6e8a943c4a9c09bafc8dc6d14bd8cc72R18.