Adding support for HTTP/2 binary headers

Motivation:

The HTTP/2 spec does not restrict headers to being String. The current
implementation of the HTTP/2 codec uses Strings as header keys and
values. We should change this so that header keys and values allow
binary values.

Modifications:

Introduced ByteString from okio (with a few minor mods). Added new
BinaryHeaders interface and made Http2Headers extend it. Various changes
throughout the HTTP/2 codec to use the new interface.

Result:

HTTP/2 codec no longer requires string headers.
Can you provide javadocs?  For example what is the return value and is its state changed?  I think the `foreach` signature typically indicates a `boolean` or an `int` (what ever makes sense) to indicate the last index or if the entire collection has been iterated over?