code cleaup. add disk_usage reporter. add handling for docs(). various tweaks and fixes

reporters/stathat.sh

@@ -4,4 +4,9 @@ report () {
   METRIC=$1
   VALUE=$2
   curl -d "stat=$METRIC&ezkey=$API_KEY&value=$VALUE" http://api.stathat.com/ez
+}
+
+docs () {
+  echo "Send data to StatHat (https://www.stathat.com)."
+  echo "\$API_KEY=<ez_key>"
 }

reporters/stdout.sh

@@ -4,4 +4,8 @@ report () {
   METRIC=$1
   VALUE=$2
   echo $METRIC: $VALUE
+}
+
+docs () {
+  echo "Print to standard output (e.g. the TTY you're running the script in)"
 }