fix issue #2480: a custom SSL context can be used for HTTPS

Fix issue https://github.com/playframework/playframework/issues/2480

The custom SSL context can be configured by the new parameter "play.http.sslcontextprovider", like this:
-Dhttps.port=9443 -Dplay.http.sslcontextprovider=myappr.MySSLContextProvider

Introduce package play.server.api for API for server concerns.