Merge pull request #313 from Coeuvre/master

Complete some easy issues
Create AUTHORS.txt
Added `AddBevelBorder` trait
Added `BevelBorderLineContext` and `BevelBorderLineColorContext`
Implemented `AddBevelBorder` for `LineContext`
Implemented `AddBevelBorder` for `LineColorContext`
Added `AddSquareBorder` trait
Added `SquareBorderLineContext` and `SquareBorderLineColorContext`
Implemented `AddSquareBorder` for `LineContext`
Implemented `AddSquareBorder` for `LineColorContext`
Update README.md
Added how to compile with library to the docs
Implemented `AddLine` for `ColorContext`
Added `BevelRectangleContext` and `BevelRectangleColorContext`
Fixed comment for `round_radius` in `RoundRectangleContext`
Update AUTHORS.txt
Updated `AUTHORS.txt`