Parasolr

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* bugfix: SolrQuerySet improved handling for Solr errors

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- Support for on-demand indexing for Django models based on signals;
see ``parasolr.django.signals``; adds a Django-specific indexable class
``parasolr.django.indexing.ModelIndexable``
- pytest plugin to disconnect django signal handlers
- Django pytest fixture for an empty solr
- Adds an EmptySolrQuerySet class, as a simpler way to check for empty results

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* ``parasolr.query.SolrQuery`` additional support for stats:

* New method ``stats`` to enable stats for a set of field names.
* New method ``get_stats`` to return the entire stats reponse.

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* ``parasolr.query.SolrQuerySet`` additional support for faceting:

* New method ``facet_field`` for more fine-grained facet feature
control for a single facet field
* New method ``facet_range`` for enabling range faceting
* Supports tag and exclusion logic via ``tag`` option on
``facet_field`` method and ``exclude`` option on ``filter``
* ``get_facets`` now returns the entire facet response, including
facet fields, range facets, etc.

* ``SolrQuerySet.filter()`` method now supports the following advanced lookups:

* **in**: filter on a list of values
* **exists**: filter on empty or not-empty
* **range**: filter on a numeric range

* New method ``SolrQuerySet.also()`` that functions just like ``only()``
except it adds instead of replacing field limit options.
* New ``parasolr.query.AliasedSolrQuerySet`` supports
aliasing Solr fields to local names for use across all queryset methods
and return values
* ``parasolr.indexing.Indexable`` now provides ``items_to_index()`` method
to support customizing retrieving items for indexing with ``index``
manage command.

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* Subquent calls to ``SolrQuerySet.only()`` now *replaces* field limit options
rather than adding to them.
* New SolrQuerySet method ``raw_query_parameters``
* SolrQuerySet now has support for faceting via ``facet`` method to configure
facets on the request and ``get_facets`` to retrieve them from the response.
* Update ``ping`` method of ``parasolr.solr.admin.CoreAdmin`` so that
a 404 response is not logged as an error.
* Refactor ``parsolr.solr`` tests into submodules

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* Fix travis-ci build for code coverage reporting.