Adding a general handler for upgrading protocols

Adding full implementation for cleartext upgrade from HTTP to HTTP/2.
Merge branch 'master' of https://github.com/netty/netty into http2

Conflicts:
	example/src/main/java/io/netty/example/http2/client/Http2Client.java
	example/src/main/java/io/netty/example/http2/client/Http2ClientInitializer.java
	example/src/main/java/io/netty/example/http2/server/Http2Server.java
	example/src/main/java/io/netty/example/http2/server/Http2ServerInitializer.java
Merge pull request #2496 from nmittler/http2

Adding a general handler for upgrading protocols