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.
@nmittler - I think this is a good time to re-visit this interface.  I was a bit suspect of one of the removal methods on the first go-around on this and was not aware the code was replicated in DefaultTextHeaders.  I think sharing the code (and then subsequently adding some tests ;) ) would be a big win.