Merge pull request #255 from NiteshKant/0.x

Updated CHANGES.md for 0.3.15
Merge pull request #256 from NiteshKant/0.x

Upgraded gradle wrapper.
Manually updating revision to 0.3.16
Fixes #260 (Convert `RxEventLoopProvider` and `MetricEventsListenerFactory` to abstract classes)
Fixes #261 (ServerSentEventEncoder is sub-optimal)

Removed usage of StringBuilder, instead uses netty's `ByteBuf`
Fixes #258 (Add port to the host header)
Merge branch '0.x' of https://github.com/ReactiveX/RxNetty into 0.x
Fixes #259 (Support native netty protocol as a runtime choice.)