Handle Multiple repositories

## A rapid first go for the dashboard to be able fetch packages from multiple repositories ; 

## Changes in the configuration
We go from old config : 
```xml
<settings>
        <repository>http://demo.exist-db.org/exist/apps/public-repo</repository>
</settings>
```
To something like: 
```xml
<settings>
    <repositories>
        <repository active="true" default="true">
            <label>exist-db public repo</label>
            <url>http://demo.exist-db.org/exist/apps/public-repo</url>
        </repository>
        <repository active="true" >
            <label>my public public repo</label>
            <url>http://localhost:8088/app/public-repo</url>
        </repository>
    </repositories>
</settings>
```

Repositories are not taken into consideration when the active **attribute= 'false'**
The default repository (exist-db public repository) need either to be the first one, or have the **default attribute set to 'true'**.

Users can see which repositories are activated at the back of eXist front icon : 
![screenshot from 2014-11-13 15 42 12](https://cloud.githubusercontent.com/assets/1168053/5030176/2768d992-6b4e-11e4-97cd-5d55817c3dcf.png)



Hope this does not break anything as it is not yet comprehensively tested  ; ) 
Cheers,
C.