Add a typescript declaration file.

This covers most of the API that is documented in the README, though I
see that there are some things exposed in `capnp.js` but not documented
-- the declaration file does not currently cover those.

This is designed to work with the output of:

    https://github.com/zenhack/capnpc-node-typescript

...which is nearing completion.

package.json

@@ -19,6 +19,7 @@
     }
   ],
   "main": "src/node-capnp/capnp.js",
+  "types": "src/node-capnp/capnp.d.ts",
   "scripts": {
     "install": "node ./build.js",
     "test": "node src/node-capnp/capnp-test.js"

src/node-capnp/capnp.d.ts

@@ -0,0 +1,35 @@
+declare module Capnp {
+  type Id = string;
+
+  abstract class StructSchema<Builder, Reader> {
+    typeId: Id;
+  }
+
+  abstract class InterfaceSchema<Server, Client> {
+    typeId: Id;
+  }
+
+  interface AnyServer {
+    close?: () => void;
+  }
+
+  interface AnyClient {
+    close(): void;
+    closed: boolean;
+    castAs<Server, Client>(schema: InterfaceSchema<Server, Client>): Client;
+  }
+
+  const Capability: {
+    new<Server, Client>(server: Server, schema: InterfaceSchema<Server, Client>): Client;
+  }
+
+  abstract class Connection {
+    restore<Server, Client>(exportName: string, type: InterfaceSchema<Server, Client>): Client;
+    close(): void;
+  }
+
+  function parse<Builder, Reader>(type: StructSchema<Builder, Reader>, buffer: Buffer): Reader;
+  function serialize<Builder, Reader>(type: StructSchema<Builder, Reader>, reader: Reader): Buffer;
+  function connect(addr: string): Connection;
+}
+export default Capnp;