Add the possibility to specify a list of targeted devices. Create two new goals : connect and disconnect

Hi everybody, 

My first Pull Request for your amazing plugin. I hope my contribution is good enough. Feedback is welcome. 

Instead of the ```<device>``` parameter, we can now specify a list of devices thanks to the ```<devices>``` parameter : 
```
<devices>
    <device>serial1</device>
    <device>serial2</device>
</devices>
```

I have also created two new goals : connect and disconnect that will run an adb connect/disconnect. 
The IP addresses will be configured  thanks to the ips parameter. 

```
<ips>
    <ip>127.0.0.1:5555</ip>
</ips>
```

Manu