Add SSL support (issue #106).

I think we are doing more than required here.
In order to provide this SslConfiguration as an abstraction over the SSLEngine, we have to replicate code from netty; `PemReader` and some of `JdkSslContext` which does not seem correct.

IMO, we should simply have someone pass `SSLEngine` to create a `SslPipelineConfigurator`.
One can use netty's `SSLContext` to create an instance of `SSLEngine` and pass it to us. If the `SSLContext` class is not "simple" for our needs, we should create facilities to simplify that but the core API (RxNetty & PipelineConfigurators) should just take an `SSLEngine`