Expose memory actor

Based on top of the webapps pull request. But only introduces the last changeset about memory actor.

It should allow watching apps memory with something like that:
```
var FirefoxClient = require("./index");
var client = new FirefoxClient();
client.connect(6000, function() {
  client.getWebapps(function (err, webapps) {
    webapps.listRunningApps(function (err, manifests) {
        manifests.forEach(function (manifestURL) {
          webapps.getApp(manifestURL, function (err, actor) {
            actor.Memory.measure(function (err, resp) {
              console.log("memory", resp);
            });
          });
         });
    });
```
Memory.measure will expose all information from nsIMemoryeporter.sizeOfTab:
  http://mxr.mozilla.org/mozilla-central/source/xpcom/base/nsIMemoryReporter.idl#381

Note that, today, devtools has a pretty huge memory footprint, but this is being tracked in following bug:
  https://bugzilla.mozilla.org/show_bug.cgi?id=1012988