Add CRAB2 lfns to Lexicon.py

Hi,

according to the email threads about this topic, I have now implemented Eric's suggestion for the CRAB 2 lfn regular expression. Hope that was the consensus?

```python
regexp4 = '/store/(temp/)*(user|group)/%(hnName)s/%(primDS)s/(%(subdir)s/)+%(root)s' % lfnParts
````

@ericvaandering: Could you review and merge that pull request, please?

Thanks,
Manuel