Issue #90 - More updates to PluginManager
Issue #90 - More updates to PluginManager

* Method 'downgrade' added to downgrade plugins
* Method 'check_for_updates' added to check for available updates
* Method 'unpin' added to unpin specified plugins
* Documented the rest

VodafoneAustralia/vha-jenkins-api-client,yp-engineering/jenkins_api_client,pc-rhuang/jenkins_api_client,VodafoneAustralia/vha-jenkins-api-client,arangamani/jenkins_api_client,cocktail-io/jenkins_api_client,Tyrael/jenkins_api_client,pc-rhuang/jenkins_api_client,pac78275/jenkins_api_client,arangamani/jenkins_api_client,stjohnjohnson/jenkins_api_client,attilastrba/jenkins_api_client,mattrose/jenkins_api_client,Tyrael/jenkins_api_client,yp-engineering/jenkins_api_client,mattrose/jenkins_api_client,attilastrba/jenkins_api_client,stjohnjohnson/jenkins_api_client,cocktail-io/jenkins_api_client,pac78275/jenkins_api_client