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.
Is the actually removing?

I'm going to hold off on reviewing the rest of this class until we decide if we can share code with `DefaultTextHeaders`.