FEATURE: send `set password` instructions after invite redemption

As soon as the user will redeem invite, they will get an email instructing them to set their password, along with password reset link:

![screen shot 2014-10-10 at 23 00 24](https://cloud.githubusercontent.com/assets/5732281/4596626/579ad174-50a5-11e4-991e-db8c56e4a19d.png)