Solr backend: correct usage of pysolr delete

We use HAYSTACK_ID_FIELD in other places but the value passed to
pysolr's delete() method must use the keyword argument ``id``:

https://github.com/toastdriven/pysolr/blob/v3.1.0/pysolr.py#L756

Although the value is passed to Solr an XML tag named ``<id>`` it will
always be checked against the actual ``uniqueKey`` field even if it uses
a custom name:

https://wiki.apache.org/solr/UpdateXmlMessages#A.22delete.22_documents_by_ID_and_by_Query

Closes #943
Fix typo with instantiating Distance units
Merge pull request #946 from gkaplan/spatial-docs-fix

Small docs fix for spatial search example code
Fix logging call in SQS post_process_results (see #648)

This was used in an except: handler and would only be executed when a
load_all() queryset retrieved a model which wasn't registered with the
index.
Moving to unified test suite, step 1, core and discovery
Removing core tests from old structure
Removing moved tests
Moved elasticsearch tests to the new test structure
Whoosh tests migrated to new structure
Make sure WhooshSearchBackendTestCase runs on different path
Moved simple tests
Moved spatial tests
Moved solr tests over
Add option for SearchModelAdmin to specify the haystack connection to use
Migrated solr to new test structure
Moved multipleindex over
Make sure we're skipping solr correctly
Move override tests
Provide a simple run script based on nose and hook setup.py test to it

Also provide requirements in setup.py
Removing remnants of old tests