Indices API: Added GET Index API

Returns information about settings, aliases, warmers, and mappings. Basically returns the IndexMetadata.

Closes #4069
Ok sounds good....as long as all the REST tests pass, I'm always afraid of any change to `PathTrie` :)
Thanks a lot for the detailed explanation! Maybe it would be better to treat it as a bugfix and get it in separately? I don't have a strong opinion though since it's something that came up with this new api and wouldn't make sense without.