Added this[string key, string defValue] accessor to Qso2, and
tests to Qso2Tests.
Added ToAdifString method to Qsos2. Added test to Qsos2Test to
test Qsos2.ToAdifString.
Fixed data type in UserdefTests for numerical field with range.
Changed Userdef.ToAdifString argument type from uint to int and check
in method that value is greater than 0.