Fix pyelasticsearch requirements; add tox

* fix pyelasticsearch requirements to allow for 0.6 and 0.6.1
* tweaks requirements/dev.txt
* adds a tox.ini to test elasticutils with python 2.6 and 2.7 on
  django 1.4, 1.5 and 1.6
Fix silly typo in CHANGELOG
Add comment about changing defaults

I think this satisfies the problems detailed in pull #171, namely
these are "constants" created at module-load time, are internal,
aren't part of the API and thus shouldn't be changed by people
using ElasticUtils.
Test for caching empty results
Cache empty results
Added support for filter and query facets.
Added tests for filter and query facets.
Added docs for filter and query facets.
[Doc] Indexing examples lacking `properties` key when configuring mappings.

The code examples for setting up mappings using both `create_index` and `put_mapping` don't include the properties key in the dict provided.
Attach facets to search result objects
Update CONTRIBUTORS
Update CHANGELOG for 0.8.2 release
Make _build_query() accept dicts as sort values
Update CHANGELOG and add Mathieu to CONTRIBUTORS