logger level methods like logger.info(...) behave the same as logger.log('info, ...)

Logger level methods like logger.info now support:
- passing metadata
```
logger.info('test message', {foo: 'bar'});
```
- interpolating string args 
```
logger.info('test message %d', 123);
```
- should simply behave as a shortcut for calling logger.log(level, ...)

It appeared that logger level helper methods like logger.info() weren't behaving the same as if I made the underlying logger.log('info', msg, metadata, callback) style call. 

I simplified the common.js setLevels to simply proxy the call to target.log, it was previously attempting to parse out callback and metadata and I first noticed that metadata was not being passed through correctly.

This pull request was initiated based on my question on StackOverflow: http://stackoverflow.com/questions/16349288/winston-js-info-method-not-passing-metadata