HTTP/2 Frame to HttpObject Adapter

Transferring @Scottmitch's personal pull request to @akamai's fork for https://github.com/netty/netty/pull/2695
This was changed to use a static construction method to avoid potential pit falls of initialization/use ordering issues. In general it is good practice to avoid using `this` (outside of a qualifier) in the constructor to avoid these types of issues.  There is an example [here](http://www.coderanch.com/t/406277/java/java/constructor) by Stan James of one way to get into trouble.