Making initial changes for svg.js v3.0

Making initial changes for svg.js v3.0

- removed `SVG.Array.split()` function (#604)
- removed workaround for browser bug with stroke-width (#560)
- removed polyfills
- removed `ungroup()` in favour of `flatten()`
- gradients now have their corresponding nodename as type and not only radial/linear (#606)
- `SVG.Path.pointAt()` correctly returns an `SVG.Point` now (#607)
- replaced static reference to `masker` in `SVG.Mask` with the `masker()` method
- replaced static reference to `clipper` in `SVG.ClipPath` with the `clipper()` method
- replaced static reference to `targets` in `SVG.Mask` and `SVG.ClipPath` with the `targets()` method (all three #563)