Align assembly verions with package release 2.0.3

Current NuGet package release is at 2.0.3 (https://www.nuget.org/packages/ExCSS/2.0.3) but the assembly version attributes still reflect version 2.0.2.
Unused references
Merge pull request #36 from atifaziz/patch-1

Align assembly verions with package release 2.0.3
Merge pull request #38 from atifaziz/unused-refs

Unused references
Updated parser blocks for closing parens inside a single value
Merge from GitHub pull requests and incrementing to version 2.0.4
Unit tests merged with passing updates to hex and hsl calculations
Merge branch 'ioninteractive-bug-background-gradients-26'