Don't error if Rails isn't defined.

This was causing errors when Rails wasn't defined:
`Uninitialized constant NotifierException::Notifier::Rails`

The easiest fix seems to be to check that Rails is defined before
trying to check if it responds to backtrace_cleaner.
Remove cattr_accesor default value, this breaks in Rails 4.0
Merge pull request #125 from dpogue/patch-1

Don't error if Rails isn't defined.
Prefer && over and to make use of short circuit evaluation.
Use `bundle exec` to setup dummy app.
fix call to #normalize_digits which was failing on background notifications
Merge pull request #130 from ghiculescu/master

Fix crash when calling #background_exception_notification
Add missing entries to Changelog.
Updated README.me - mention ActionMailer setting

Added to the documentation that a default ActionMailer 
delivery_method must be set for emails to be sent. 
Added instructions for it.
Merge pull request #134 from anandvc/master

Updated documentation to include ActionMailer setting without which emails will not work.
Merge branch 'smartinez87/exception_notification'