Integrate dexd with iov-core

[ethanfrey]

For a nice user interface, we need javascript bindings that work in the browser. Luckily, we have 90% of what we need in the iov-core library.

We need to show how to build a new package like iov-bns from the protobuf definitions, and expose the particular new transaction types and queries through iov-core interfaces. This may also trigger some refactoring of iov-core to make the weave codecs easier to reuse.

End result is a simple javascript interface for all dexd blockchain applications. But more importantly, documentation for how to build such a binding