Refactor node_exporter to support collectors.

A collector is a type matching 'Collector' interface.

The following collectors where added:
- NativeCollector wrapping the original functionality (attributes, load)
- GmondCollector scraping ganglia's gmond (based on gmond_exporter)
- MuninCollector scraping munin (based on munin_exporter)

exporter/helper.go

@@ -0,0 +1,26 @@
+package exporter
+
+import (
+	"fmt"
+	"log"
+	"strconv"
+	"strings"
+)
+
+func debug(name string, format string, a ...interface{}) {
+	if *verbose {
+		f := fmt.Sprintf("%s: %s", name, format)
+		log.Printf(f, a...)
+	}
+}
+
+func splitToInts(str string, sep string) (ints []int, err error) {
+	for _, part := range strings.Split(str, sep) {
+		i, err := strconv.Atoi(part)
+		if err != nil {
+			return nil, fmt.Errorf("Could not split '%s' because %s is no int: %s", str, part, err)
+		}
+		ints = append(ints, i)
+	}
+	return ints, nil
+}