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.
oof ... called the wrong method.  Good catch! :)

I've noticed that the test coverage is pretty low for both this test and DefaultTextHeaders.  Combining into a common base class would definitely make testing easier, but it may require changing the TextHeaders interface a bit.  Is that something we can do at this point?