change logger interface to not return errors (Refs #50)

See https://github.com/go-kit/kit/issues/164 for discussion of why
kitlog returns an error.

```
Package log is designed to be used for more than simple application info/warning/error logging; it's suitable for log-structured data in an e.g. Lambda architecture, where each invocation is important. I agree with you that if we were doing only application logging the error would be more noise than signal. But the scope of the package is larger than that.
```

Since we are doing only application logging and we're not checking
errors, it is safe to get rid them.

log/filter_test.go

@@ -2,7 +2,6 @@ package log_test
 
 import (
 	"bytes"
-	"errors"
 	"strings"
 	"testing"
 
@@ -71,23 +70,6 @@ func TestVariousLevels(t *testing.T) {
 	}
 }
 
-func TestErrNotAllowed(t *testing.T) {
-	myError := errors.New("squelched!")
-	opts := []log.Option{
-		log.AllowError(),
-		log.ErrNotAllowed(myError),
-	}
-	logger := log.NewFilter(log.NewNopLogger(), opts...)
-
-	if want, have := myError, logger.Info("foo", "bar", "baz"); want != have {
-		t.Errorf("want %#+v, have %#+v", want, have)
-	}
-
-	if want, have := error(nil), logger.Error("foo", "bar", "baz"); want != have {
-		t.Errorf("want %#+v, have %#+v", want, have)
-	}
-}
-
 func TestLevelContext(t *testing.T) {
 	var buf bytes.Buffer