Fetch description from gemspec

closes #1

I added a first approach of implementation for handle the cases when the gems are not in rubygems.org

this is specified with the options at the time of declare a gem in a gemfile, for example with

```ruby
gem 'pry', github: 'pry/pry', branch: 'development'
```

because the `:github` option was provided, I fetch the gemspec from github and then extract the *summary*

@jgorset I would like to review the code and make comments before merging this pull request