Add Access-Control-Allow-Origin header

I've been using TumblrThemr these past few days, and although not an ideal solution, it has been helpful.

One thing I noticed when using it is that Chrome will not allow access to the Tumblr XML to JSON service if the page is not loaded via a webserver; accourdingly, the browser provides an `Origin` header value of `null`, and the request is denied.

Though I'm admittedly not fully versed on CORS protocol, in theory, simply adding a

```
Access-Control-Allow-Origin: *
```

header will alleviate this problem, and allow those not so technically minded to use TumblrThemr within browsers that adhere to CORS.

This is what is added in this PR, though I am not familiar with Ruby or the Sinatra web framework, this is based purely off http://stackoverflow.com/questions/4199624/how-do-i-set-http-headers-in-ruby-sinatra-app-hosted-on-heroku