Update MapReduce JSON Parsing

While adding client support for M/R jobs with bucket types, I found the code comments a little behind the times.  So the `parse_inputs/1` error string now shows alternate usage with optional bucket types.  This will hopefully make maintenance easier in the future.  I also found that if the `default` bucket type is specified, Riak does not like this.  So now if `<<"default">>` is specified, it will be parsed as if no bucket type had been specified giving the desired behavior.  Tests for the new parsing behavior were also added.