Add support for GPX endpoints

There are several  [GPX endpoints](https://wiki.openstreetmap.org/wiki/API_v0.6#GPS_traces) in the OSM API, e.g.:

```
GET /api/0.6/trackpoints?bbox=left,bottom,right,top&page=pageNumber
```

or

```
POST /api/0.6/gpx/create
```

osmapi should support those endpoints.

Related: #102 