do not use kwarg for unicode errors
[#8] Update spatial harvester docs for 2.0
[#8] Fix code tags
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