Add ability to pass in a logger

[josegonzalez]

Would be great if we could pass in a Logger somehow to perform logging. This would allow users of the library to change the logging to match what they have within their applications.

The alternative is setting the standard logger via:

import "log"

...

log.SetFlags(0)
log.SetOutput(someLogger)

But the log output doesn't contain a level at that point because of the log.Printf calls (so I guess it would also be nice to change those to something that is a level...).