sidekiq in specific project not in enigine

Engine initializers are executed before RoR application is started.
As a conclusion we are not able to pass any parameter from RoR application
into engine initializer.
coverage report to git ignore
configurable ability class

The main idea is to separate ability model between main application and
atmosphere. If user does not want to extend model and REST interface than
user is not forced to create ability class. Than `Atmosphere::Ability`
will be used. If ability modification are needed than:

1. Ability class should be created e.g.

```ruby
class Ability < Atmosphere::Ability
  ... #additional abilities
end
```

2. Ability class should be set in Atmosphere setup

```ruby
Atmosphere.setup do |config|
  config.ability_class = '::Ability'
end
```
security_proxy_id removed from allowed params