Add support for selecting collectors at buildtime

This works by using a global array with references to NewXCollector
functions. Each collector appends to that array in it's init() function.

Which file gets build depends on the build tags:

To build only the ganglia exporter, you can do:

    go build -tags nonative,ganglia

By default it will build only the native collector.

node_exporter.go

@@ -2,8 +2,9 @@ package main
 
 import (
 	"flag"
-	"github.com/prometheus/node_exporter/exporter"
 	"log"
+
+	"github.com/prometheus/node_exporter/exporter"
 )
 
 var (
@@ -18,5 +19,9 @@ func main() {
 	if err != nil {
 		log.Fatalf("Couldn't instantiate exporter: %s", err)
 	}
+	log.Printf("Registered collectors:")
+	for _, c := range exporter.Collectors {
+		log.Print(" - ", c.Name())
+	}
 	exporter.Loop()
 }