Initialize instance variables fixes #58

This should fix all the errors when trying to use all the tasks that were causing uninitialized variable errors. I did some debugging as well and looks like the response is the same with or without these variables initialized when it works. For instance debugging

    def self.manage_timer(response)
      logger.debug response.headers['x-shopify-shop-api-call-limit'].split('/')
      @@current_api_call_count, @@total_api_calls = response.headers['x-shopify-shop-api-call-limit'].split('/')
      @@current_timer = Time.now if @current_timer.nil?
    end

Results were:

    $ theme upload assets/app.css.liquid
    DEBUG -- : [1,40]
    $ theme replace
    DEBUG -- : [1,40]
    DEBUG -- : [1,40]
    DEBUG -- : [1,40]
    DEBUG -- : [1,40]