make the welcome email more semantic

most mail agents will [strip out](http://kb.mailchimp.com/article/css-in-html-email) `<style>` tags.

Here's what the sign up email looked like via gmail ![gmail screenshot](http://screens.launchacademy.com/e35eb949c3a58415b8ce0f5fa8f19629.jpg)

For now, I made the markup a bit more semantic to make it look more attractive short term when this happens. Long term, we should swap out the `<style>` tags for inline styles. With full CSS support, it now looks like this:

![html screenshot](http://screens.launchacademy.com/b5172c5ae06e5266477e63dbdc943bed.jpg)

I'd recommend a mailer layout with inline styles and the use of a tool like [roadie](https://github.com/Mange/roadie) or [premailer](https://github.com/premailer/premailer) (my preference is premailer)