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.
My intention was to use a dash "-" to identify the source of the "user_import" and "user_password". I imagine there may be other sources for this types of user imports that will each need their own Mandrill templates. But yes, the convention is to only use underscores "_". Just trying to expand the convention.