Indices API: Added GET Index API

Returns information about settings, aliases, warmers, and mappings. Basically returns the IndexMetadata.

Closes #4069
I think you raise a good point.  The problem with this change at the moment is that it only exposes the functionality on the REST layer and seems a bit messy with the reuse of the cluster state API.  I think I will expose it as a proper java API as you suggest, putting it more in line with the other endpoints in the Indices API.  That way the API can basically expose the IndexMetaData object which contains all the info including the warmers which have to be retrieved separately in this method.