Stop folder when running out of disk space (fixes #2057)

& tweaks by calmh

Godeps/_workspace/src/github.com/calmh/du/diskusage.go

@@ -0,0 +1,8 @@
+package du
+
+// Usage holds information about total and available storage on a volume.
+type Usage struct {
+	TotalBytes int64 // Size of volume
+	FreeBytes  int64 // Unused size
+	AvailBytes int64 // Available to a non-privileged user
+}