Merge pull request #371 from Coeuvre/master

Removed `Vec2d` and `Ray`
Removed `Triangle`
Added `Ray`, `Vec2d` and `Polygon` to `vecmath`
Using type alias for `Polygon` in `PolygonContext`
Using type alias for `Polygon` in `AddPolygon`
Using type alias for `Polygon` in `Context`
Using type alias for `Polygon` in `ColorContext`
Added `Polygons` type alias
Using type alias for `Polygons` in `TweenContext`
Using type alias for `Polygons` in `AddPolygons`
Using type alias for `Polygons` in `TweenColorContext`
Using type alias for `Polygons` in `TweenPolygonsContext`
Using type alias for `Polygons` in `TweenPolygonsColorContext`
Made code consistent with type aliasing
Merge pull request #373 from bvssvni/type_alias

Going back to type alias
Added `get_scale` to `vecmath`
Merge pull request #375 from bvssvni/get_scale

Added `get_scale` to `vecmath`
Update links to PistonDevelopers
Merge pull request #376 from gmorenz/fix-readme

Update links in readme to PistonDevelopers
Moved `Image` to its own file.