Indices API: Added GET Index API

Returns information about settings, aliases, warmers, and mappings. Basically returns the IndexMetadata.

Closes #4069
@javanna 
The fix is to solve an issue introduced with this new API. This API binds to {index}/{feature} where feature is _settings, _mappings, _warmers, or _aliases, but we also have other APIs which bind to e.g. {index}/_settings/{type} etc. the settings API doesn't have a handler for {index}/_settings but before this change would have been chosen and ES would return an error saying there is no handler for {index}/_settings when actually we want it to pick the handler for {index}/{feature} in this case.

This fix now means that if we are requesting a/b and there is a handler for a/* and a handler for a/b/c , but no handler for a/b, the a/* handler will be chosen rather than returning an error.