Add std package level logger

[aloababa]

I think having a package level logger is handy in many cases, most of the time you will want a logger that prints on stdout at the info level. So you don't have to instanciate a new logger every time.

I had to rename log level constants by adding Level prefix because it was clashing with log package level function like Debug, Info, Warning... So i may introduce some breaking changes if users were using those log level constants.

I also added some set methods to configure the logger afterwards.