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 we need to add a unit test case around this.  I suspect there is a bug according to the description of the method from `BinaryHeaders` and `TextHeaders`:

>Returns and removes the value of a header with the specified name.  If there are
 more than one values for the specified name, the first value is returned.

I think this is actually removing all entries with the same name and returning the value of the last entry.  This loop is very similar to the loop from `remove0` which is supposed to remove all entries.