Put mailer code in Mailer service class
Use Mailer service to send e-mail, implement Responder pattern
Refactor Mailer to use external HTML mail templates
Normalize e-mail handling for clients and invoices
Change position of E-mail buttons for Client#show
Don't show Reminder and Send Invoice buttons at the same time
Add test for testInvoicesOfClient
Fix method typo
Fix failing JSON test
Fix failing toggleStarred test
Merge branch 'email' into develop