added exactly_one_of validation

updated readme

updated changelog

make exactly_one_of inherit from mutually_exclusive, add test

fixed typo, small changes
Merge pull request #637 from Morred/exactly_one_of

added exactly_one_of validation
Wrong date in CHANGELOG.
Added support for blocks with reusable params.
Use a specific RBX version, attempt at fixing CI.
Just build latest rbx-2.