Feature/pagination

Added pagination support for the following resources:
- `/vms`
- `/run`
- `/dashboard`

The implementation is made via 2 parameters: `offset` and `limit`
On the reply there is 3 attributes that allow to render a pagination footer: `offset`, `limit`, `count`.
On all of these resources you can filter by Cloud service with the parameter `cloud`.
On the `/run` resource you can find Runs for a specific module with the parameter `moduleResourceUri`.

**Example:**
GET on https://slipstream.sixsq.com/run?media=xml&offset=10&limit=5
```xml
<vms offset="10" limit="5" count="19">
    <vm cloud="CloudC" user="super" instanceId="4f2708e1-6aa6-4469-8f5e-b18ffd42cb50_machine" state="Running" measurement="2014-12-08 13:09:24.82 CET" runUuid="a1b345f0-c434-490e-849f-c3894af55588"/>
    <vm cloud="CloudC" user="super" instanceId="157701e9-9c52-45b1-a506-13241446aad3_machine" state="Terminated" measurement="2014-12-08 13:09:24.98 CET" runUuid="a1b345f0-c434-490e-849f-c3894af55588"/>
    <vm cloud="CloudA" user="super" instanceId="vm_gateway" state="Running" measurement="2014-12-08 13:09:24.112 CET" runUuid="35c3789f-5da1-4504-8294-d41489d035ae"/>
    <vm cloud="CloudA" user="super" instanceId="vApp_6_centos" state="Running" measurement="2014-12-08 13:09:24.126 CET" runUuid="Unknown"/>
    <vm cloud="CloudB" user="super" instanceId="1b0afd4f-340d-4bef-89e2-d6f4a776f2ea" state="Running" measurement="2014-12-08 13:09:33.215 CET" runUuid="Unknown"/>
</vms>
```