Indices API: Added GET Index API

Returns information about settings, aliases, warmers, and mappings. Basically returns the IndexMetadata.

Closes #4069
I think you exposed too many options :) these two are not exposed to the REST layer, are kinda hidden options that we use internally... look here: https://github.com/elasticsearch/elasticsearch/blob/master/src/main/java/org/elasticsearch/action/support/IndicesOptions.java#L147