Adding functionality to not post commit status for children jobs

[Eriten]

Updated Amrit's PR #309 with better code organization and DSL support.

To quote what Amrit describes about what this PR does,

It adds the ability to allow the user to not post commit statuses for the child job of a parent matrix job. If the checkbox is for this function is not pressed, it will follow its default behaviour.
I've added a checkbox in the config.jelly file that saves the value in "showMatrixStatus". The GhprbSimpleStatus class will read the value of the field. If the value is false it will create a commit status and proceed as normal (!showMatrixStatus). If the value is true, then will check if the project is a matrix project. Since the children jobs are not Matrix projects, we can avoid posting commit statuses for them by checking both the value of the box and if it is an instance of a matrix project (showMatrixStatus && build.getProject() instanceof MatrixProject)