Upsert support from Hive

Here's a modification I've been using for running upsert from hive. Basically, create an upsert-enabled hive table using

'es.index.write.strategy'='upsert',
'es.id.path'='<your id>'
  
I also used @barnybug's _id extraction from https://github.com/elasticsearch/elasticsearch-hadoop/pull/53.

Don't worry too much about this implementation, but I thought you'd find it useful when you consider implementing the real thing. I've tested this quite extensively with millions of documents and lots of upserts on top of each and it's been super convenient.

Cheers!!