Exceptions

Implemented exception logging as part of grohl.  [Heavily inspired by scrolls](https://github.com/asenchi/scrolls/blob/5f97cb90aeb3aae68b1f5fd061b9b03c49e17717/lib/scrolls/log.rb#L95-L118), as usual.  

First, create an exception logger.  If you don't specify a reporter, it will use the default reporter that logs through grohl.

```go
err := fmt.Errorf("Sample Error")

reporter := grohl.NewExceptionLogger(nil)
reporter.Report(err, grohl.LogData{"app": "sample"})
```

/cc @asenchi