Properly handle short-form plugin descriptions for plugins without labels
Print a newline between plugin short form descriptions

Otherwise a `jpm search git` looks like a big wall of text
Fix a stupid issue when running `jpm list`

On a real machine I found this stupid exception:

    ubuntu@ip-10-226-175-124:/vagrant/pkg$ jpm list
    /var/lib/gems/1.8/gems/jpm-1.0.0/lib/jpm.rb:32:in `has_plugins?': undefined method `<=' for #<Array:0x7fbb402b3240> (NoMethodError)
            from /var/lib/gems/1.8/gems/jpm-1.0.0/lib/jpm/cli.rb:11:in `list'
            from /var/lib/gems/1.8/gems/thor-0.19.0/lib/thor/command.rb:27:in `__send__'
            from /var/lib/gems/1.8/gems/thor-0.19.0/lib/thor/command.rb:27:in `run'
            from /var/lib/gems/1.8/gems/thor-0.19.0/lib/thor/invocation.rb:126:in `invoke_command'
            from /var/lib/gems/1.8/gems/thor-0.19.0/lib/thor.rb:359:in `dispatch'
            from /var/lib/gems/1.8/gems/thor-0.19.0/lib/thor/base.rb:440:in `start'
            from /var/lib/gems/1.8/gems/jpm-1.0.0/bin/jpm:14
            from /usr/local/bin/jpm:19:in `load'
            from /usr/local/bin/jpm:19
Add simple (early) support for `jpm update` to fetch and write out an update-center.json

Tested this on a physical machine with no pre-existing update-center.json, wooo

References #6