Added support for multiple webservers.

The webserver now uses its name to define the service prefix. This means you can run multiple webservers in parallel to show different things, or hosted on different machines, but within the same ros system.

Default behaviour remains unchanged, i.e. launched with:

```
rosrun strands_webserver strands_webserver
```

To run an extra server in parallel on port 8091 with service prefix /bob
```
rosrun strands_webserver strands_webserver __name:=bob _port:=8091
```