3.1 Release

(including pod）

1. Refactored some components to better align with latest Swift language trend by jjatie.
2. For bug and PRs, please check changelog, or https://github.com/danielgindi/Charts/projects/6
3. New unit tests for pie chart.
4. Support Xcode 9.2 and Swift 4.0.3

We are making Swift refactoring recently and target for 4.0 release along with some other features, thanks everyone for improving this project!