Prefer ELASTICSEARCH_URL environment variable as a default URL, if present

Note: Main reason is frictionless deployment at <Heroku.com>

Closes #240, closes #241
Added "text" query as part of the DSL [#168]

Example:

    Tire.search('articles-test') do
      query { text :title, '+1' }
    end

See <http://www.elasticsearch.org/guide/reference/query-dsl/text-query.html>
Improved code structure, added unit test and separated integration tests for the "text" query implementation

Closes #168.
Prepare release 0.3.13 (pre-release version)
[FIX] Added encoding header for DSL example file

(Reported as #257)
[FIX] FilteredQuery: chain multiple filters with `and` by default

Closes #254, closes #255.
[FACETS] Added support for "statistical" facets [#252]

See http://www.elasticsearch.org/guide/reference/api/search/facets/statistical-facet.html
[FACETS] Added "term_stats" facets [#252]

* adding terms_stats facets support
* removing unnecessary tests from statistical

See http://www.elasticsearch.org/guide/reference/api/search/facets/terms-stats-facet.html
[FACETS] Code/tests cleanups for "terms_stats" and "statistical" facets

Closes #252.
[PERSISTENCE][FIX] Changing instance properties should not affect property default
Hash and Array... were passed to property as reference, not object
Added a `Search#version` method to control returning the `version` field in hits

Closes #190
[FIX] Only retry in `Index#bulk_store` on StandardError, not Exception

Otherwise bulk_store retries after receiving signals like INT or TERM,
likely resulting in the process getting killing by the KILL signal without
executing normal cleanup (e.g. ensure blocks).

Closes #223.
[FIX] Fixed showing the incorrect percentage when importing via Mongoid

Tested with Mongoid 2.4.1

Closes #232
Pre-release 0.4.0.pre
[DEPRECATE] Deprecate the support for passing JSON strings to `Index#store` [#275]
[ACTIVEMODEL] Support any level of mappings in `mapping`

Previously, only two levels of mappings were supported, ex:

    mapping do
      indexes :title, :type => 'string'
      indexes :author do
        indexes :first_name, :type => 'string'
        indexes :last_name,  :type => 'string', :boost => 100
      end
    end

This commit adds support for any level of mappings, ex:

    mapping do
      indexes :title, :type => 'string'
      indexes :author do
        indexes :first_name, :type => 'string'
        indexes :last_name,  :type => 'string', :boost => 100

        indexes :posts do
          indexes :title, :type => 'string', :boost => 10
        end
      end
    end

Closes #210.
[DOC] Update information for installing ElasticSearch to the 0.19.0 version
[DOC] Update documentation for Mongoid users in Readme

* Leave out needless code
* Use `as_json`, not `to_json` in the `Article.to_indexed_json`!
* Use the proxy to not mix it up with Mongoid interface: `Article.tire.search 'love'`

Fixes #157.
Added the `:raise` option to Index#bulk_store and Index#import

When the `:raise` option is passed, any exception received is re-raised.

Closes #224.
[#224] Cleaned up the Index#import interface and import/bulk_store tests

* Removed the positional argument `method` and moved it into the options Hash
* Updated tests
* Cleaned up mocked interfaces in Index tests