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:
Hi @ConradIrwin.
It's a cool thing to replace description by filenames as sometimes descriptions can be too long and I never liked this(some of my old gists)

so, I changed it to show file names. It works like this
```
>> gist -l pravj
[public gists]
https://gist.github.com/2f500607d0aa1bae26a2 : ["leaderboard.coffee"]
https://gist.github.com/e4af092f494bd46cddc9 : ["99BottlesOfBeer.swift"]
https://gist.github.com/d39fb4149638c183b922 : ["treegen.sh", "use.md"]
https://gist.github.com/ca794865262fb441a226 : ["HelpNet.md"]
```
the case where more then one files are there, also included.