Move the mailing list subscription box to homepage (Fixes #560)

I've just created a mailing list signup box on the frontpage.

![sidebar](https://f.cloud.github.com/assets/373438/296424/7853e7fe-94f7-11e2-9fb5-283c4e479262.png)

@anderspeders Let me know if this looks good to you.

Fixes #560