Add Plugins#__str__

Because when doing jenkins.get_plugins() in an interactive python shell,
the behavior was to use the JenkinsBase#__str__ which throws a
NotImplementedError
ensure virtualenv installed and WORKSPACE defined
Fetch full build list when incomplete

By default, Jenkins returns the last 100 builds. If there are more
builds, an extra api call is made to have all builds in the job object
Merge pull request #175 from Smartesting/fetch_all_builds

Fetch full build list of job when incomplete