Fix build.get_revision() for GIT repositories

Filter out actions which are None or not of interest and then extract the git revision from the lastBuiltRevision. Works in cases where branch is of something other than origin/HEAD
jenkins_launcher can now install plugins

install_plugin takes in a URL or a list of URLs and restarts jenkins thereby installing the plugins
Begin test cases for testing jenkins API calls related to the SCM

Use JenkinsLauncher.install_plugin to install Jenkins git plugins and verify coverage for Build.get_revision() fix
Use requests instead of urllib.urlretrieve for plugins