Convert all of web-ext promise chains to coroutines

[kumar303]

All code that currently builds chains of promises would be way more concise using co. Let's rewrite it!

Example of rewriting test code:

it('zips a package', () => withTempDir(
  (tmpDir) =>
     co(function*() {
       let zipFile = new ZipFile();

       let {extensionPath} = yield adapter.buildMinimalExt(tmpDir);
       assert.match(extensionPath, /minimal_extension-1\.0\.xpi$/);

       yield zipFile.open(extensionPath);

       let fileNames = [];
       let result = yield zipFile.readEach((entry) => fileNames.push(entry.fileName));

       assert.deepEqual(fileNames, ['manifest.json']);

       return result;
     })
));

[kumar303]

I'm not totally sure if this is possible in babel since we'd like to support older system nodes and also running without a --harmony flag. Some things to dig into:

• http://babeljs.io/docs/plugins/syntax-async-functions/
• http://babeljs.io/docs/plugins/transform-regenerator/

[kumar303]

We totally did this in #411 !