Move class methods into ClassMethods module for better extensibility.

My uses case is something along the lines of this.

```ruby
# /lib/segment_io.rb
module SegmentIO
  CONFIG = OpenStruct.new(enabled: false)
  extend AnalyticsRuby::ClassMethods

  def self.config(&block)
    yield CONFIG if block_given?
    SegmentIO.init(secret: CONFIG.secret) if CONFIG.secret && CONFIG.enabled
  end
end

# /config/initializers/segment_io.rb

SegmentIO.config do |config|
  config.enabled  = true
  config.secret   = ENV["SEGMENT_PRODUCTION_SECRET"] || 'dev-test-secret'
  config.api_key  = ENV["SEGMENT_PRODUCTION_APIKEY"] || 'dev-test-apikey'
end

# call it server side like.
SegmentIO.track(user_id: '53', event: 'Test Event')

# Also reference the api_key client side for javascript
# <% SegmentIO::CONFIG.api_key %>
```

This pull is to support the...
```ruby
...
extend AnalyticsRuby::ClassMethods
...
```
...bit above.