Random prefixes for blank nodes

## Problem

I want to use raptor to be able to concatenate RDF files. At this moment, the only reason why this is not possible is because blank nodes are given the same identifiers when converting different files.

## Suggested solution

Giving random prefix names per file. This way we can use something like 
```bash
ls *.rdf | while read file ; do { rapper -o ntriples $file } done > dump.n3
```
to concatenate a directory of rdf files in one dump.