Changed 'xsd:int' occurrences in the README to match 'xsd:integer'

Changed 'xsd:int' occurrences in the README to match 'xsd:integer' in the ontology. #8

Needed for representing extremely large genomes such as found in amoeba. See "Amoeba proteus" in http://mbe.oxfordjournals.org/content/25/4/787.full