Support grpc-web

The representation of gRPC requests and message data on the wire in the `httpgrpc` package is quite different from that used to by `grpc-web` (which also aims to support gRPC on an HTTP 1.1 channel).

To converge on a compatible approach, the `httpgrpc` package should seamlessly support the same wire format details as `grpc-web` (possible "v2" of its wire format?). That will allow the server side to actually just use an [existing grpc-web implementation for Go](https://github.com/improbable-eng/grpc-web), and it also means the Go client could be used with other grpc-web server implementations.

Supporting this new format would also be an ideal time to address #20 (which requires a change to the on-the-wire representation of a gRPC request anyway).