Move platform specific gems to top of gemfile

Selecting gems with if statements does not work.
Bundler supports a platform command, but for 
now it's easier and more clear to just 
comment out non-osx dev stuff.
Use a mock user object in email preview

Security issue.
Update to latest Premailer syntax for strings
Improve handling of application.yml and env vars
Check for ALLOW_EMAIL_PREVIEW
Update email preview section of README