Missing nullValue element in DescribeCoverage

[warmerdam]

The DescribeCoverage result should be extended to include null values when
available from the GDAL datasource. 

It appears that the NullValues element is coverage wide ... not specific to 
a particular band as it is in GDAL.  We would likely need logic to only return
a  NullValues element if all bands have a matching null value. 

I'll take this bug myself as time permits.

[warmerdam]

I have implemented support for returning the NullValues in DescribeCoverage and actually setting the output image background to this value and returning the null value setting within the data file where supported (r10099). Also documented (r10098) and added tests (r10100).