Indices API: Added GET Index API

Returns information about settings, aliases, warmers, and mappings. Basically returns the IndexMetadata.

Closes #4069
this is not needed, it should only be done for actions that were supported in 1.4 since we changed all the action names recently. If you get test failures because of it, I guess ActionNamesTests, you just need to add the api to the list of the ones to be ignored [here](https://github.com/elasticsearch/elasticsearch/blob/master/src/test/java/org/elasticsearch/transport/ActionNamesTests.java#L125)