Bump version to 2.6.1

Bump package versions to 2.6.0

Remove unnecessary dependencies for the Audio module

Generate NuGet packages on build

Fixed addition/subtraction for Vector3f

Merge pull request #111 from SFML/feature/2.3_prep

Bumped version number to 2.3 and updated readme.txt

Merge pull request #103 from zsbzsb/update/version_number

Updated version number to 2.2

Updated version number

Updated the website links in readme.txt

Updated CSFML libraries