Support indexed scripts API #979

Hello,

I've added three new descriptors to handle put, get and delete indexed scripts.
ElasticClient has been extended to handle new descriptors.
Alos, I've changed ScriptFilterDescriptor. Now, this class can use indexed script id.

I've wrote unit and integration tests to check new feature.

Would be great to change type of parameter lang, from string to ScriptLang enum, but it'll require change in ElasticsearchPathInfo class. Do you think it's fine?