Add GRIB file with missing values.

Needed to test the handling of missing values in GRIB messages for [this PR](https://github.com/SciTools/iris/issues/520). The file is around 14 Kb in size.

I'm not 100% sure how this data complies with usage policies that are in place. It is a field extracted from a file downloaded from  http://www.nco.ncep.noaa.gov/pmb/products/gfs/. I didn't see any obvious usage restrictions listed on that site, so I assume this data is in the public domain.