Handle relation in XML

Hi,
Since I really needed relations for my project, I did it myself.

Every relation has this form:

```
"tags": {
    "except":"bicycle",
    "restriction":"only_straight_on",
    "type":"restriction"
},
"members":{
    "nodes":[],
    "ways":[{"ref":"238239023","role":"to"},{"ref":"8019802","role":"from"},{"ref":"35297466","role":"via"}]},
    "id":"105752",
    "visible":true,
    "version":3,
    "changeset":19774307,
    "user":"Xinfe",
    "uid":"167945"
}
```

Hope you enjoy it!