added error and, fixed tests to yo polymer:el and corrected yo polyme…

[luxedo]

• Added an error when running yo polymer:el outside a PSK root
• Fixed yo polymer:element by creating a symbolic link
• Fixed tests in for yo polymer:el

The elements.html file was being created inside

before(function (done) {
      helpers.run(path.join(__dirname, '../app'))

but was deleted inside:

it('creates expected files', function () {
      var expected = [
        'app/elements/my-foo/my-foo.html'
      ];
      assert.file(expected);
    });

raising the new error in el/index.js

To address the issue, instead of creating the /app data using the helper, I suggest to create a dummy elements.html file just before the tests begin

.on('ready', function(generator){
          fs.createFileSync(path.join(__dirname, './tmp/app/elements/elements.html'));
        })