Solr backend: run() kwargs are passed directly to search backend

This allows customization by subclasses and also fixes #888
by ensuring that the custom field list prepared by 
`ValuesQuerySet` and `ValuesListQuerySet` is actually used.
ElasticSearch backend: run() kwargs are passed directly to search backend

This allows customization by subclasses and also fixes #888
by ensuring that the custom field list prepared by 
`ValuesQuerySet` and `ValuesListQuerySet` is actually used.
Tests: skip Solr content extraction with old PySolr

Until pysolr 3.1.1 ships there's no point in running the Solr content
extraction tests because they'll fail:

https://github.com/toastdriven/pysolr/pull/104
Merge pull request #888 from acdha/888-solr-field-list-regression

Solr / ElasticSearch backends: restore run() kwargs handling

This fixes an earlier regression which did not break functionality but made `.values()` and `.values_list()` much less of an optimization than intended.

#925 will be a more comprehensive refactor but this is enough of a performance win to be worth including if a point release happens before #925 lands.
Tests: update pysolr requirement to 3.1.1

3.1.1 shipped a fix for a change in the Solr response format for the
content extraction handler
Tests: remove legacy targets

* Django 1.4 is no longer supported as per the documentation
* Travis: use Python 3.3 targets instead of 3.2
Ignore missing index during .clear()

404 in indices.delete can only mean that the index is there, no issue
for a delete operation

Fixes #647
Ignore 404 when removing a document from elasticsearch

Fixes #942
Add a note on elasticsearch-py versioning with regards to 1.0
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