added support for parent-child relations

Our logs use [relations](http://www.elasticsearch.org/blog/managing-relations-inside-elasticsearch/) to manage hierarchical logging types. This requires us to use the `_parent` field of the bulk post api.

I have modified the plugin to allow a configurable parent field. It is used identically to the id_key config param. I was able to re-use the test cases for id_key as they are identical to the behaviour of parent_key.