Indices API: Added GET Index API

Returns information about settings, aliases, warmers, and mappings. Basically returns the IndexMetadata.

Closes #4069
I think I mislead you here. I meant to add support for indices options to the new api, not to cluster state api directly (which I'm not sure about and if done without version checks would also break bw comp). Have a look at the existing get settings api for instance or get aliases. The difference is that you are exposing a new REST api only (no transport action), thus you would need to call concrete indices yourself on the REST action. Now that I think about it though I'm wondering if it would make sense to expose this get index as a proper java api too, like get settings and co. What do you think?