WebSearch: fix record numbering for pagination

* Adds the nb_found argument to print_records allowing for a different
  list of records to be printed and number of records found, when we are
  only printing a subset of a total number of records found.

(closes #1762)

Signed-off-by: Nikolaos Kasioumis <nikolaos.kasioumis@cern.ch>
We may want to be more defensive here.  If `nb_found` is not passed, what about setting it from `len(recIDs)`?  Otherwise any old code calling `print_records()` without `nb_found` would stopped working.  Example:

```python
import cStringIO
tmp = cStringIO.StringIO()
print_records(req=tmp, recIDs=range(1,3), format='xm')
out = tmp.getvalue()
tmp.close()
```

Proposed fix: change default argument value to `nb_found=-1`, and:

```python
if nb_found == -1:
    nb_found = len(recIDs)

if nb_found:
```

If OK, with you, I'll amend merge that way.