Added Nested Sort

[dam-bal]

• Added Sorting interface that all sorts can implement.
• Added Concerns to Sort for missing, unmapped_type and mode (https://www.elastic.co/guide/en/elasticsearch/reference/current/sort-search-results.html#_sort_mode_option).
• Simplified toArray in Sort, also now supports mode and is implementing Sorting interface.
• Changed SortCollection and Builder to use Sorting interface instead of Sort.
• Added tests for Sort.
• Added NestedSort (https://www.elastic.co/guide/en/elasticsearch/reference/current/sort-search-results.html#nested-sorting)
• Updated readme to include info about NestedSort.
• Removed ExampleTest.php.

[AlexVanderbist]

Thanks! We'll tag this as a new major and provide some notes about the updated Sort/Sorting interface in the UPGRADING.md.