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)

node_exporter.conf

@@ -1,4 +1,10 @@
 {
+  "scrapeInterval": 10,
+  "collectors": [
+    "NativeCollector",
+    "GmondCollector",
+    "MuninCollector"
+   ],
    "attributes" : {
       "web-server" : "1",
       "zone" : "a",