Check if jenkins_home/plugins exists

The code was raising an exception if ~jenkins
existed, but ~jenkins/plugins didn't. Change
the code to check for plugins from jenkins_home.
lint fix
Add note about plugin dependencies to README.markdown
Merge remote-tracking branch 'upstream/master'
Merge pull request #83 from jchristi/master

Add note about plugin dependencies to README.markdown
Merge pull request #82 from dcrissman/dev

Lint Fix
Merge pull request #68 from ambakshi/fix-jenkins-plugins-facter

Check if jenkins_home/plugins exists
Merge remote-tracking branch 'upstream/master'