Update welcome email to emphasize calendar export

@mduan Review please?

Here's our old welcome email:

![image](https://f.cloud.github.com/assets/159687/2496121/61b1c226-b311-11e3-842a-b99cfe5d1827.png)

Now that we have calendar export, this updates it to emphasize that feature.

- Change blurb to say "Did you know that you can [upload your class schedule](https://uwflow.com/profile?import-schedule=1)? This lets you export it to Google Calendar and iCal."
- Update screenshot to this one:

![image](https://uwflow.com/static/img/class-schedule-screenshot-nov-2013.png)

Not tested yet.