Improved listing output / presentation

Just a quick modification to the output format so that it looks the same as the one used by `pod search`. Also added support for the `--verbose` flag to add the authors to the output:
```
-> Pod Dependencies
   Shows a project's CocoaPod dependency graph.
   - Gem:      cocoapods-dependencies
   - URL:      https://github.com/segiddins/cocoapods-dependencies
   - Author:   Samuel E. Giddins
```

NB: I also fixed some coding convention in the code, like using single-quoted strings when possible.

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Notes about this PR:

1. This is my very **first pull request on ruby code** (I'm a ruby beginner), so please review!
2. I was **unable to execute `rake` to run the RuboCop tests** before submitting this PR (probably due to sthg obscure specific to my ruby environment?)… It didn't seem to me that the tests were to be adapted to the new output format but I couldn't test it . I'll wait for Travis to tell me!
3. But I **successfully installed the gem** (`gem build cocoapods-plugins.gemspec && sudo gem install cocoapods-plugins-0.1.1.gem`) and **successfully executed both `pod plugins` and `pod plugins --verbose` commands** and everything seemed to work as expected.