Support parsing of string with multiple identifiers

I've been doing some work with the [RIS](http://en.wikipedia.org/wiki/RIS_(file_format)) format and have had all sorts of weird and worderful interpretations of how to record identifiers for documents.  To support this (as it's not going away in a hurry) I've added a method (StdNum::Helpers. extract_multiple_numbers) for extraction of multiple identifiers based on the StdNum::Helpers.extractNumber.

So what we get is the following:
```ruby
string = '9780987115423 (print ed) 9780987115430 (web ed)' 
array_with_matches = ['9780987115423', '9780987115430']
```

Let me know what you think