Separate cluster helper methods to ClusterHelperMethods
Add lazy load hook
Fixes #16 Initialize clusters and install helper methods after configuration loaded

In lazy loading environments, clusters were not initialized.
this causes helper methods(i.e. xxx_cluster_transaction,
all_cluster_transaction) to raises errors.
Merge pull request #17 from drecom/issue/16-fix-cluster-support-methods-when-lazy-loading

Issue/16 fix cluster support methods when lazy loading