s1734: added User and Group JSON reader/writers

projects/cadcAccessControl/build.xml

...adcAccessControl/src/ca/nrc/cadc/ac/json/GroupReader.java

...adcAccessControl/src/ca/nrc/cadc/ac/json/GroupWriter.java

...cadcAccessControl/src/ca/nrc/cadc/ac/json/UserReader.java

...cadcAccessControl/src/ca/nrc/cadc/ac/json/UserWriter.java

...l/test/src/ca/nrc/cadc/ac/json/GroupReaderWriterTest.java

...ol/test/src/ca/nrc/cadc/ac/json/UserReaderWriterTest.java

...src/ca/nrc/cadc/ac/xml/GroupPropertyReaderWriterTest.java

...ol/test/src/ca/nrc/cadc/ac/xml/GroupReaderWriterTest.java

...l/test/src/ca/nrc/cadc/ac/xml/GroupsReaderWriterTest.java

...test/src/ca/nrc/cadc/ac/xml/IdentityReaderWriterTest.java

...rol/test/src/ca/nrc/cadc/ac/xml/UserReaderWriterTest.java