Show changelogs on install

Hello, again.

I added `--changelog` option to `gem install/update` commands, which shows changelogs on install/update.
This branch is based on [`llines` branch][lines].

After merging this patch, you can show changelogs on install/update:

    $ gem install gem-changelog --changelog
    Successfully installed gem-changelog-1.0.2
    ## 1.0.2 (2013-03-05)
    
    - Add "History" to the changelog patterns.
    - Update CHANGELOG.md
    
    ## 1.0.1 (2013-03-05)
    
    - Code cleanup
    
    ## 1.0.0 (2013-03-05)
    
    Done installing documentation for gem-changelog (0 sec).
    1 gem installed

To specify the number of lines to be shown, you may pass it to the option:

    $ gem install gem-changelog --changelog 4
    Successfully installed gem-changelog-1.0.2
    ## 1.0.2 (2013-03-05)
    
    - Add "History" to the changelog patterns.
    - Update CHANGELOG.md
    
    Done installing documentation for gem-changelog (0 sec).
    1 gem installed

You can set `--changelog` and specify the number of lines by writing/adding to `.gemrc` file such like `--no-document` or `--no-ri --no-rdoc`:

```yaml
gem: --changelog 20
```

Could you consider to accept this pull request, please?

[lines]: https://github.com/sakuro/gem-changelog/pull/2