added support for setting log level using strings: SetLogLevel()

A clean pull request for `SetLogLevel(level string)`, which adds setting the loglevel using strings. Makes it easier to set using configuration settings.

"warn" and "dbg" added as alternates to "warning" and "debug", respectively. 

Assumes lowercase.

Test cases added.
As mentioned in the previous PR, please remove `dbg` since it's used nowhere else in Logrus.