Map boost field to _boost in elasticsearch.

Means that including a boost field in a document will cause document level boosting.
Update the current elasticsearch boost test to also test document boosting.
Started porting to py3.
More porting to Py3.
Core tests mostly pass on Py 3. \o/

What's left are 3 failures, all ordering issues, where the correct output is present, but ordering is different between Py2 / Py3.
Closer on porting ES.
Whoosh all but fully working under Python 3.
Simple backend ported & passing.
Overrides passing under Py3.
Elasticsearch tests passing under Python 3.

Requires git master (ES 1.0.0 beta) to work properly when using suggestions.
Update autocomplete.rst
Fix datetime instantiation in elasticsearch backend's _to_python
Add test__to_python for elastisearch backend
Added ethurgood to AUTHORS.
Merge pull request #744 from trigger-corp/master

Allow for document boosting with elasticsearch
Merge pull request #821 from knightzero/patch-1

Update autocomplete.rst
Solr tests all but passing under Py3.
Final core changes.
Backported things removed from Django 1.6.
Whoosh mostly working under Python 3. See docs for details.