Configurable sender for notification emails

Do it like JIRA:

1. Specifies the From: header format in notification emails. Default is of
2. the form "John Doe (JIRA) <jira@company.com>".
3. Available variables are ${fullname}, ${email} and ${email.hostname}
4. If commented out, format is just 'jira@company.com'.
   jira.email.fromheader.format = ${fullname} (JIRA)