Added support for passing partial document to `Index#update`

    Tire.index('articles-with-tags') do
      update( 'article', '1', :doc => { :title => 'One UPDATED' } )
    end

See <http://www.elasticsearch.org/guide/reference/api/update.html>

Requires Elasticsearch version [0.20](http://www.elasticsearch.org/download/2012/12/07/0.20.0.html) or higher.

Fixes #366.
Added `nested` query support

* Added the `nested` implementation for API/DSL
* Added unit and integration tests

Closes #475
Added examples of `nested` query support into `tire-dsl.rb`
[#475] Cleaned up and simplified the tests for `nested` query

* Fix whitespace errors
* Proper mapping in the `NestedQueryTest`
* Simpler queries in tests
* Better names for test cases, reorganize
* Remove redundant tests
[#475] Refactored the documentation for `nested` query in `tire-dsl.rb`