Logging of request tags to dumpfile

As already suggested in https://support.process-one.net/browse/TSUN-266 I added logging of the specified request tag to the `tsung.dump` file (on `dumptraffic="protocol"`).

See https://support.process-one.net/browse/TSUN-269.

```xml
<session name="request-tag-example" probability="100" type="ts_http">
  <request> <http url="/" method="GET" version="1.1"></http> </request>
  <request tag="meh"> <http url="/meh" method="GET" version="1.1"></http> </request>
</session>
```

will yield in:

```
#date;pid;id;http method;host;URL;HTTP status;size;duration;transaction;match;error;tag
1382017950.138046;<0.92.0>;1;get;localhost;/;200;72;53.401;-;;;
1382017950.161481;<0.92.0>;1;get;localhost;/meh;200;75;23.198;-;;;meh
```

### TODOs

- [x] change the tag default value to `'undefined'` (atom)
- [x] documentation
- [ ] add tests?