Add ability to access session_id from ActiveZuora::Connection

This could go a step further and add file download as a customization on the Export object.

Example usage;

	  export = ActiveZuora::Export.new
	  export.name = "Test"
	  export.format = "csv"
	  export.zip = true
	  export.query = "SELECT * FROM subscription"
	
	  url = "https://www.zuora.com/apps/api/file/#{export.file_id}"
	
	  id = ActiveZuora::Export.connection.session_id
	
	 Net::HTTP.start(uri.host, uri.port) do |http|
	    request = Net::HTTP::Get.new(uri.request_uri)
	    request.add_field("Authorization", "ZSession #{id}")
	
	    http.request(request) do |response|
	      response.read_body do |chunk|
	        src.write(chunk)
	      end
	    end
	  end

http://knowledgecenter.zuora.com/D_Zuora_APIs/SOAP_API/C_SOAP_API_Reference/H_Export_ZOQL
http://knowledgecenter.zuora.com/D_Zuora_APIs/SOAP_API/C_SOAP_API_Reference/C_API_Use_Cases_and_Examples/I_Creating_an_Export