qualifies default notifier as EmailNotifier

rename ExceptionNotifier::Notifier class to ExceptionNotifier::EmailNotifier
isolate middleware options
drop campfire integration
extracts Mailer from EmailNotifier
Refactor ExceptionNotification to accept notifiers

The ExceptionNotifier middleware nows accepts multiple notifiers.

Notifiers are objects which respond to method `#call(exception, options)`
in order to execute some action: send an email, publish to a chat room,
call a webhook or whatever your crazy little mind wants!

You can configure your notifiers passing a hash of options to the
ExceptionNotifier middleware in the given form:

    use ExceptionNotifier,
      :email => { ... email options ... },
      :campfire => { ... campfire options ... },
      ...

Though, to this thing work, notifiers MUST BE implemented under the
ExceptionNotifier namespace and MUST HAVE a name ending 'Notifier'. e.g:
'ExceptionNotifier::EmailNotifier'.

Moreover, any notifier can be added/removed later in some place of
the application, like a Rails initializer, using the method
'ExceptionNotifier#register_exception_notifier(name, notifier)' since
the `notifier` responds to `#call(exception, options)` method. Example:

    ExceptionNotifier.register_exception_notifier :test,
      lambda { |exception, options| ... do something ...  }

By now, the only notifier implemented is EmailNotifier.
reintegrate campfire notifier
add webhook notification

send notifications via HTTP
apply changes from recent commits
maintain support for old API
update CHANGELOG
Merge branch 'smartinez87/exception_notification'
Merge pull request #139 from jweslley/master

make exception_notification a generic notification driver