Add plain text parts to email

Summary:
Added plain text templates
Added tests for plain text email
Fix so \n\n is a new paragraph in HTML output

Test Plan:

./phpunit
php artisan template:render Mail/NegativeBalance --text
# use this with all templates, run php artisane template:render help to get a list

Reviewers: #hkccp_developers, vanmeeuwen

Reviewed By: #hkccp_developers, vanmeeuwen

Subscribers: #hkccp_developers

Differential Revision: https://git.kolab.org/D1354