Display gem summary when verbose is passed into show command

Add summary option to 'show' cli command. Displays the gem's summary if present, or reverts to description.

`bundle show --summary`

```
Gems included by the bundle:
  * actionmailer (2.3.2) - This is just a fake gem for testing
  * actionpack (2.3.2) - This is just a fake gem for testing
  * activerecord (2.3.2) - This is just a fake gem for testing
  * activeresource (2.3.2) - This is just a fake gem for testing
  * activesupport (2.3.2) - This is just a fake gem for testing
  * bundler (1.6.0.pre.2)
  * rails (2.3.2) - This is just a fake gem for testing
  * rake (10.0.2) - Ruby based make-like utility.
```