Allow extended-format mapping files to be used for beliefs instead of just simple dictionaries.

This allows mappings generated by metamap (metamap.ioos.us) with full descriptions, eg:

```
{
  "comment": {
    "query": "/ncml:netcdf/ncml:attribute[@name='comment']/@value",
    "desc": "Miscellaneous information about the data.\t"
  },
  "time_coverage_resolution": {
    "query": "//ncml:attribute[@name='time_coverage_resolution']/@value)",
    "desc": "Describes the temporal coverage of the data as a time range.\t"
  },
  "geospatial_vertical_max": {
    "query": "/ncml:netcdf/ncml:attribute[@name='geospatial_vertical_max']/@value",
    "desc": "Describes a simple latitude, longitude, and vertical bounding box. For a more detailed geospatial coverage, see the suggested geospatial attributes.\n"
  },
```