Elasticquery

+ Fix for timeout: append 's' on the end (changable via `time_type` kwarg)

+ Add `Query.constant_score`
+ Add `ElasticQuery.filter` and `ElasticQuery.query` as options instead of using `.must`, `.should` & `.must_not`
+ Add `ElasticQuery.timeout`
+ Tidy up `ElasticQuery.offset` and `ElasticQuery.limit`
+ Creates query structure on demand (empty query is now properly empty)

+ Add `execution` to terms filter

+ Remove field checking with `ElasticMapping` (incompatible with nested fields, needs rethink)

+ Add `match` filter + query
+ Add `range` aggregation

+ Add `reverse_nested` aggregation