add LogErrorf/LogWarnf/LogInfof/LogDebugf for formatter logging

using fmt.Sprint instead of fmt.Sprintln for logging print
fixed missing engine.LogWarnf replacement
Merge remote-tracking branch 'origin/master' into feature/cacher-tag
add Engine.LogSQLQueryTime and Engine.LogSQLExecutionTime for SQL execution time logging
redis cacher implementation, incomplete
Update session.go

fix stmt query of 'Get function' bug when an error occurred
Merge pull request #151 from starchou/master

Update session.go
bug fixed for mssql limit
Merge branch 'master' of github.com:go-xorm/xorm
Merge remote-tracking branch 'origin/master' into feature/cacher-tag