Merge branch 'v2' into issue-90

db.go

mongo/benchmark_test.go

mongo/database.go

mysql/benchmark_test.go

mysql/database.go

mysql/database_test.go

mysql/tx.go

postgresql/benchmark_test.go

postgresql/database.go

postgresql/database_test.go

postgresql/tx.go

ql/benchmark_test.go

ql/database.go

ql/database_test.go

ql/tx.go

sqlite/benchmark_test.go

sqlite/database.go

sqlite/database_test.go

sqlite/tx.go

util/sqlutil/sqlutil.go