Beta release of v3.0.0-beta.10

• Improve modular rendering, in part to support vega-webgl-renderer.
• Add Node v7 support, drop official support for Node v0.12.
• Add change method for data modification to View class.
• Add fastmap utility. Update stateful operators to use fastmap, perform garbage collection.
• Fix aggregate distinct operator bug.
• Fix Scale rangeStep calculation bug for point scales.