Adds support for niche.com transaction types

Fixes #16 

Adds support for niche.com transaction types:
- user_welcome-niche: General welcome email to niche.com users
- case user_password-niche: email informing the user of their temp DoSomething user password.
Ah, yea, I figured that's what you were going with here. Yea, I don't know what the ideal is. It's probably fine as long as we continue with that convention going forward.