[#15] Add suport for Solr spatial indexing and querying

When the 'ckanext.spatial.search_backend' config option is set to
'solr', the extension will index geometries stored in the 'spatial'
extra on the spatial field of the Solr index (named 'spatial_geom').
This is done on the 'before_index' extension point.

Also, when doing a query, if the same config option is in place, the
necessary fq parameter will be set to pass the spatial query to Solr.
[#8] Fix bug that prevented setting a default resource name
[#15] Ensure that bounding boxes are defined counter-clockwise

To return correct results on a spatial query, rectangle geometries must
be defined in counter-clockwise order [1]. This changeset adds a small
sanity check to before_index when we are dealing with a Polygon geometry
that has 5 coordinate pairs. Shapely is used to generate a LinearRing
from the polygon coordinates and check if they are ccw. If not, they are
reordered and a new polygon is generated so the WKT sent to Solr is
properly ordered.

The GeoJSON template used for extents in the base spatial harvester has
been also updated to define the coordinates counter-clockwise.

[1]
http://wiki.apache.org/solr/SolrAdaptersForLuceneSpatial4#JTS_.2BAC8_WKT_.2BAC8_Polygon_notes
Merge branch 'release-v2.0' into 15-solr-based-spatial-search
Merge branch 'release-v2.0' into 15-solr-based-spatial-search
[#15] Read search backend config option once only
[#15] Add a new Solr search backend based on boost function

This backend does not use the spatial field on Solr, but indexes the
area and coordinates of the dataset's extent. On query time, a boost
function is used to calculate a ratio between the query area and the
dataset extent, which will give a value of 0 for areas that don't
overlap and 1 for a perfect fit. The same function is used both for
sorting and filtering.
[#15] Document different spatial search backends available
[#15] Tweak doc to remove paper
[#8] Don't add object id to error so it can be aggregated
Merge branch '15-solr-based-spatial-search' into release-v2.0
Merge branch 'release-v2.0' of github.com:okfn/ckanext-spatial into release-v2.0