Race condition when accessing loggers

[dignifiedquire]

The global variable in oldlog.go creates race issues when trying to call Logger("name") from two different threads.

[Stebalien]

This is usually to be called during initialization. That is:

var log = logging.Logger("my-module-name")

However, it shouldn't cost anything to make it thread-safe.

[Stebalien]

Yeah, we'll want this anyways for pluggins.

[Stebalien]

However, you should still avoid doing this outside of init.

[Stebalien]

Fixed in #44.