Introducing the CSP Fixer

@devoncarew

Devon: sorry, a couple of unrelated trivial changes have crept in.

I should also add tests.

Also note that I deliberately exposed the feature only in the form of an explicit action. I didn't want to build it into the Bower install/update sequence or anything like that because it's too early and too risky.

Further explanation to the comment above: the output HTML looks like this:

```html
<html><head><link rel="import" href="../core-iconset/core-iconset.html">

<link rel="stylesheet" href="core-icon.css" shim-shadowdom="">

</head><body><polymer-element name="core-icon" attributes="src icon alt">
<script src="core-icon.html.0.js"></script>

</polymer-element>
</body></html>
```
 
Note the `<html>`, `<head>` and `<body>` tags that are atypical to how Polymer elements are usually written. To my surprise, this nevertheless worked, so I didn't bother beautifying this further.