[DBClientUser] Add a new method for chaning the base class.
[DBClientUser] Use DBAgent::transaction() for one line operations.
[DBClientUser] Use DBAgent::transaction() in addUserInfo().
[DBClientUser] Use DBAgent::transaction() in updateUserInfo().
[DBClientUser] Use DBAgent::transaction() in deleteUserInfo().
[DBClientUser] Use DBAgent::transaction() in addUserRoleInfo().
[DBClientUser] Use DBAgent::transaction() in updateUserRoleInfo().
[DBClientUser] Shorten conditions of if statements
[DBClientUser] Remove an unused variable
Merge pull request #398 from project-hatohol/trx-user

Use DBAgent()::transaction instead of DBCLINET_TRANSACTION_BEGIN() macro.
The previous statement with the marco has broken the C++ syntax grammar and
seems to a little storange.