[fixed JENKINS-19740] Added postCheckout for SCMs

This pull request will first of all add an additional method for the SCM extension point, letting SCM implementations do work after the change log is parsed and written. Secondly it will provide functionality to solve https://issues.jenkins-ci.org/browse/JENKINS-19558. 

JENKINS-19558 describes a scenario where the change log is not written even though there is a change set. This is also the case with the git plugin.