OL3-Cesium 1.8 release


I am pleased to announce version 1.8 of OL3-Cesium,
based on OL 3.8.0 and Cesium 1.13.

This version brings many new features and improvements:
- a plugin system to allow building your code together with the library;
- more extensibile synchronizers to allow custom logics like raster to vector
- uncoupled Cesium.js so that it may be loaded lazily by the application;
- reworked synchronizers to allow zIndex handling and fix a blue flash which
  could occur when changing layers.

Thank you to all contributors of OL3, Cesium and OL3-Cesium for making this
release possible.

As announced earlier, I will give a talk with Andreas at FOSS4G Seoul next week;
looking forward to meeting you there.