Refactor architecture creation
- Update version to 0.2.15-SNAPSHOT
- Refactor CNNArch2MXNet: Make architecture symbol creation reusable
- Refactor LayerSupportChecker: LayerSupportChecker is now an interface which can be implemented by child generators like CNNArch2Gluon to state which layer is supported
- Add a concrete implementation of LayerSupportChecker that allows all layers