Avoid unnecessary db query on index update

pks_seen is only needed if objects are removed from
index, so only compute it if necessary.
Improve pks_seen to not build an intermediary list.
Fix documentation regarding ModelSearchIndex to match current behavior

Closes #606. Thanks to @bradleyayers for the report.
Remove bit about SearchQuerySet.load_all_queryset deprecation

That method was entirely removed in commit b8048dc0e9e3.

Closes #607. Thanks to @bradleyayers for the report.
Merge pull request #881 from benspaulding/docs/issue-606

Fix documentation regarding ModelSearchIndex to match current behavior
Merge pull request #882 from benspaulding/docs/issue-607

Remove bit about SearchQuerySet.load_all_queryset deprecation
Fixed ValueError exception when SILENTLY_FAIL=True
Added pabluk to AUTHORS
fixed typo in AUTHORS
Cleaner query composition when it comes to filters in ES
Use terms filter for DJANGO_CT, *much* faster
Merge remote-tracking branch 'HonzaKral/filtered_queries'
Fixes #886.
update travis config so that unit tests will run with latest solr + elasticsearch
Use HTML5 <input type=search> (closes #899)
Merge pull request #899 from acdha/html5-input-type=search

Search form <input type="search">
Use elasticsearch-py instead of pyelasticsearch.

elasticsearch-py is the official Python client for Elasticsearch.
Merge pull request #885 from HonzaKral/elasticsearch-py

Use elasticsearch-py instead of pyelasticsearch.
upgraded python3 in tox to 3.3

3.3 is a better target for haystack than 3.2, due to PEP414
Merge branch 'master' into avoid-pks-seen-on-update

Change smart_text into smart_bytes as in master

Conflicts:
	haystack/management/commands/update_index.py
Merge pull request #775 from stefanw/avoid-pks-seen-on-update

Avoid unnecessary, potentially huge db query on index update
Merge branch 'master' into update-index-language