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.

README.md

@@ -1,4 +1,31 @@
-node_exporter
-=============
+# node_exporter
+
+Prometheus exporter with plugable metric collectors.
+
+
+
+## Available collectors
+
+By default it will only include the NativeCollector.
+
+To include other collectors, specify the build tags accordently.
+
+
+### NativeCollector
+
+Provides metrics for load, seconds since last login and a list of tags
+read from `node_exporter.conf`.
+
+To disable the native collector, use build tag `nonative`.
+
+
+### GmondCollector
+
+Talks to a local gmond and provide it's metrics.
+
+
+### RunitCollector
+
+Provides metrics for each runit services like state and how long it
+has been in that state.
 
-Simple exporter, exposing load, seconds since last login and a list of tags for the node it's running on.