Upgrade to Rails 4.0.1

I ran `rails new money-tracker` from this app's parent directory and
then went through each of the changed files and manually merged their
contents where applicable.

I've had to disable turbolinks (it was added to application.js by
default) to avoid the 'Editing multiple transactions with javascript'
test from failing. Everything appeared to work OK in development even
with turbolinks enabled.
Avoid deprecation warning with default scope

The warning was:

DEPRECATION WARNING: Calling #default_scope without a block is
deprecated. For example instead of `default_scope where(color: 'red')`,
please use `default_scope { where(color: 'red') }`.
Fix broken exception notifier config

Prior to this, I was seeing the following error in the Heroku logs:

    undefined method `new' for ExceptionNotifier:Module (NoMethodError)

The info on upgrading is in the Exception Notifier README[1].

[1] https://github.com/smartinez87/exception_notification#upgrading-to-4x-version
Add Heroku rails_12factor gem

Apparently this is required for a Rails 4 app to serve assets in
production[1].

[1] http://stackoverflow.com/questions/15354539/heroku-does-not-compile-files-under-assets-piplines-in-rails-4
Merge branch 'upgrade-to-rails-4'

All the tests pass and I've tested that it correctly deploys to Heroku.