➕ Add dependency on inherits

At the moment, running `Backend` in the browser requires a polyfill of
the entire `util` built-in, when we only use `inherits`.

This change adds a dependency on `inherits`, which [uses][1] the Node.js
built-in if available, otherwise [falls back][2] to a browser-compatible
polyfill.

This allows downstream consumers avoiding having to polyfill `util`.

[1]: https://github.com/isaacs/inherits/blob/1286b65f07a03953d3857bf3dd1d77e8beede704/inherits.js#L5
[2]: https://github.com/isaacs/inherits/blob/1286b65f07a03953d3857bf3dd1d77e8beede704/inherits.js#L8

lib/op-stream.js

@@ -1,4 +1,4 @@
-var inherits = require('util').inherits;
+var inherits = require('inherits');
 var Readable = require('stream').Readable;
 var util = require('./util');
 

lib/stream-socket.js

@@ -1,5 +1,5 @@
 var Duplex = require('stream').Duplex;
-var inherits = require('util').inherits;
+var inherits = require('inherits');
 var logger = require('./logger');
 var util = require('./util');
 

package.json

@@ -8,6 +8,7 @@
     "async": "^3.2.4",
     "fast-deep-equal": "^3.1.3",
     "hat": "0.0.3",
+    "inherits": "^2.0.4",
     "ot-json0": "^1.1.0"
   },
   "devDependencies": {