[modules] Add support for disk utilization

Add a new module "disk" that takes an optional parameter (the path) and
displays free & total disk space, along with the usage percentage.

Also, added Tunnel/VPN support to the themeing of the "net" module.

bumblebee/util.py

@@ -0,0 +1,8 @@
+
+def bytefmt(num):
+    for unit in [ "", "Ki", "Mi", "Gi" ]:
+        if num < 1024.0:
+            return "{:.2f}{}B".format(num, unit)
+        num /= 1024.0
+    return "{:05.2f%}{}GiB".format(num)
+