Include occurrenceID in query results from occurrencelist()

These two commits modify the function gbifxmlToDataFrame() such that it also extracts the information on a field referred to by the GBIF API as gbifKey. This field is more properly referred to as occurrenceID based on the definition supplied here: https://code.google.com/p/darwincore/wiki/Occurrence#occurrenceID.  The benefit of this field is that you can use this number to look up any record returned from a particular query, for example if the occurrenceID is 145637212 then you can use the following webaddress to retrieve information on that specific record:

http://data.gbif.org/ws/rest/occurrence/get/145637212

Note that in practice the occurrenceID's are supposed to be static; however, it appears that this is not always the case: http://iphylo.blogspot.com/2012/07/dear-gbif-please-stop-changing.html