Support for PutMapping, Update and IndicesExist APIs

Hi,

I added support for PutMapping (_mapping API http://www.elasticsearch.org/guide/en/elasticsearch/reference/current/indices-put-mapping.html). Should be good, please check it!

Also added Update API support. See http://www.elasticsearch.org/guide/en/elasticsearch/reference/current/docs-update.html for details. Dynamic scripting is disabled by default on ElasticSearch servers from something like 1.2.X, so there is an error handling and skipping implementation for cases such as that.

Also, i added support for HEAD methods, they return no body, only HTTP status code. This made any HEAD request like http://www.elasticsearch.org/guide/en/elasticsearch/reference/current/indices-exists.html fail.
I actually needed the HEAD thingy to be able to implement the above linked IndicesExist API. So, you can also find that implementation among the commits.

Update: Added GetMapping API. Since this API returns fully dynamic response, I had to add Raw member on Response which is unmarshalled whole response from ES server. 

Thanks
Marin