HTTP/2 Draft 15

Motivation:
A new draft of the HTTP/2 spec has been released.

Modifications:
Make updates as defined in https://tools.ietf.org/html/draft-ietf-httpbis-http2-15

Result:
HTTP/2 codec is draft 15 compliant.
@Scottmitch just to keep the options simpler, how about canning the extra methods for specific error types?  I think we can get by with just 4 methods:
```java
public static Http2Exception connectionError(Http2Error error, String fmt, Object... args);
public static Http2Exception connectionError(Http2Error error, Throwable cause, String fmt, Object... args);
public static Http2Exception streamError(int streamId, Http2Error error, String fmt, Object... args);
public static Http2Exception streamError(int streamId, Http2Error error, Throwable cause, String fmt, Object... args);
```
WDYT?