Layer attribute values enumeration service

Prototype/demo for a webservice listing the values of a given attribute from a given layer (useful in the QueryBuilder plugin to build combos in the condition panels).

This require in the project part to:

- list the DBSession objects created in the project. For instance:
```
# in <project>/models.py
from c2cgeoportal.models import DBSessions
DBSessions.update({ 
    'dbsession2': DBSession2,
    'dbsession3': DBSession3,
    'dbsession4': DBSession4,
    'dbsession5': DBSession5,
    'dbsession6': DBSession6,
    'dbsession7': DBSession7,
    'dbsession8': DBSession8,
    'dbsession9': DBSession9,
})
```

- configure the layer/attribute info in the main ``config.yaml.in``. For instance:
```
  layers_enum:
      areal_liegenschaften:
          dbsession: 'dbsession7'
          attributes:
              gemeinde:
                  table: 'np_areal.v_p_areal_liegenschaften'
                  fieldname: 'gemeinde'
```

- config in viewer.js:
```
        attributeURLs: {
            "areal_liegenschaften": {
                "gemeinde": "${request.route_url('layers_enumerate_attribute_values', layer_name='areal_liegenschaften', field_name='gemeinde', path='')}"
            }
        },
```