Add a JavaScript SDK for Tezos.

This SDK shall implement typical commands that one runs using the Tezos client to call Tezos node RPCs, but in JavaScript. The goal is for this SDK to be complete enough to be usable by someone who would want to build a complete Tezos wallet.

Another goal is for this SDK to be easy to maintain and to update when new RPCs are added to the node.

One way to achieve those goals is to compile part of the client using Js_of_ocaml.