logging interface

[xiang90]

save log to remote place.

[xiang90]

• define logging levels
• define output: stdout/stderr
• remote logging: map stdout/stderr to local/remote files

[xiang90]

@fengjingchao @xiaoyunwu feel free to modify/add more.

[xiaoyunwu]

So the main goal is the all the output from every task from this distributed app should be saved for some time so that one can inspect the content to figure out what actually happened. Just so it is clear.