output for jenkins_command

I added an output attribute for  `jenkins_command` so we can capture and parse the stdout that is returned by a command.

```ruby
command = jenkins_command 'who-am-i' do 
notifies :create, 'ruby_block[test]'
end

ruby_block 'test' do 
  block do
   Chef::Log.info command.output
  end
end

```
will log a message like `Authenticated as: jenkins\n Authorities:`

I did this because I was trying to get the output of commands like `get-job` 

```ruby
command = jenkins_command 'get-job template'
```
will return in `command.output` a whole xml file that can be used with `jenkins_job` to create other jobs using templates stored and managed by jenkins