added SetLogLevel to allow setting the loglevel using a string

First thing I did when implementing logrus was look for a way to easily set the level from a string, since that is the user friendly way of adding the configuration setting for loglevel. This request was also in issue #56.

I added SetLogLevel, which takes a string and calls SetLevel() using the constant associated with that level. I also added variants for error, warn, and debug, so that its handles some other common values for those keywords, lowering friction to the library user.

Tests were added for the new function.
I would prefer a `ParseLevel` func, alike to `time.ParseDuration`. Otherwise this works only for the package global logger.