[DB MIGRATION REQ'D] Twitter configure and crawl public accounts

# Introduction

Twitter lets you get information about other users who are not protecting their tweets.
With Twitter REST API you can get the same information for others like for you except mentions ans retweets.
So, with adding a public Twitter account, you can configure other Twitter accounts to crawl it.

## User interface

### How to add an account ?
* First you need to set up the Twitter Plugin
* Next you need to set up an authorized Twitter account
* Then, admin only, you can add a public Twitter account in Settings > Plugins > Twitter 
> Configure > Add a public Twitter account

![image](https://f.cloud.github.com/assets/570816/2021460/8f6d61c2-883f-11e3-9c41-5dddd473810e.png)

### How to see which ones are public ?

Public account appear under **Public Account** title:

![image](https://f.cloud.github.com/assets/570816/2021478/cae72850-883f-11e3-99c1-058474704206.png)

## Twitter crawl

We have added **screen_name** argument to API calls:
* followers/list
* friends/list
* users/show

When twitter account to crawl is not authorized we can't make this calls:
* statuses/mentions_timeline
* statuses/retweets_of_me

## Database Migration Required

We have altered table **tu_owner_instances** adding a new column named **is_twitter_referenced_instance**


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