0.1.3 - 2015-04-01
  * bug fixed: raised error if Settings.using_slave is nil/false and using block
0.1.2 - 2015-03-31
  * support new usage:
      ActiveRecord::Base.using(:slave_name) do
      end
  * prevent usage: ActiveRecord::Base.using(:slave_name).where()
  * add error classes
0.1.1 - 2015.03.31
  * alias using using_slave
  * bug fixed: 父查询中包含default_scope等子查询时导致父查询不走从库
  * supprt includes/pluck/count/sum etc.