implements feature to list all gists

it works in this way : 

`gist --list` or `gist -l` : will check if user is authenticated or not
> if user is authenticated : will list all `public` and `private` gist urls with description also
> if user is not authenticated : warn user to login, using `gist --login` and exit.

it also takes one `optional parameter` as `username`
`gist --list abhshkdz` will list all public gists of user `abhshkdz`, it resulted : 
```
[public gists]
https://gist.github.com/7460904 : 
https://gist.github.com/7276887 : Generating a Url slug in PHP
https://gist.github.com/5916998 : Music Widget: http://cdpn.io/zvGqf
https://gist.github.com/5879343 : Sublime Text 2 Installation Instructions for Ubuntu 13.04
https://gist.github.com/4151814 : Self-updating list of my tweets
https://gist.github.com/4148037 : Source nvm + tab completion
https://gist.github.com/3940532 : Links Thread
https://gist.github.com/3325927 : Building up Hubot on Gtalk (deployed on Heroku)
```

@ConradIrwin @rking can you please see this, as it was discussed in issue #168 also
tell me if I need to do anything else :hand:
actually this block executes when user is not authenticated and trying to use it like a authed user
`gist --list`(without providing username)

So, this is just a message to user for help. I can add `exit 1` explicitly if you say.
but when I put `raise Error, MESSAGE` here. It results in
```
/var/lib/gems/1.9.1/gems/gist-4.2.1/lib/gist.rb:190:in `pretty_gist': Bad credentials (RuntimeError)
	from /var/lib/gems/1.9.1/gems/gist-4.2.1/lib/gist.rb:141:in `list_gist'
	from /var/lib/gems/1.9.1/gems/gist-4.2.1/bin/gist:114:in `block (2 levels) in <top (required)>'
	from /usr/lib/ruby/1.9.1/optparse.rb:1391:in `call'
	from /usr/lib/ruby/1.9.1/optparse.rb:1391:in `block in parse_in_order'
	from /usr/lib/ruby/1.9.1/optparse.rb:1347:in `catch'
	from /usr/lib/ruby/1.9.1/optparse.rb:1347:in `parse_in_order'
	from /usr/lib/ruby/1.9.1/optparse.rb:1341:in `order!'
	from /usr/lib/ruby/1.9.1/optparse.rb:1432:in `permute!'
	from /usr/lib/ruby/1.9.1/optparse.rb:1453:in `parse!'
	from /var/lib/gems/1.9.1/gems/gist-4.2.1/bin/gist:13:in `<top (required)>'
	from /usr/local/bin/gist:23:in `load'
	from /usr/local/bin/gist:23:in `<main>'
```
which does not look fine.

can you comment here, please. @ConradIrwin