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 - The `Text` and `Ascii` naming scheme is a bit confusing but it seems like it is similar enough to the ByteString.  We should add a `forEach` method in `AsciiString` to keep a consistent iteration scheme and avoid potentially having to check the index on every access.