Merge pull request #327 from Coeuvre/master

Updated `AUTHORS.txt`
Add `AddBevel` trait
Implemented `AddBevel` for `BevelRectangleContext`
Implemented `AddBevel` for `RectangleColorContext`
Implemented `Clone` for contexts when possible
Implemented `Clone` for `PolygonContext` and `PolygonColorContext`

Decoupled lifetime of polygon from the field
Implemented `Clone` for `TweenPolygonsContext` and `TweenPolygonsColorContext`
Made some functions to inlined