api: Various fixes.

- limit list buckets to limit only 100 buckets, all uppercase buckets
  are now lowercase and work transparently with all calls.
- Change disk.Stat to disk.GetInfo and return back disk.Info{} struct.
- Introduce new ioutils package which implements ReadDirN(path, n),
  ReadDirNamesN(path, n)

pkg/disk/disk.go

@@ -16,11 +16,11 @@
 
 package disk
 
-// StatFS stat fs struct is container which holds following values
+// Info stat fs struct is container which holds following values
 // Total - total size of the volume / disk
 // Free - free size of the volume / disk
-// FSType - file system type string
-type StatFS struct {
+// Type - file system type string
+type Info struct {
 	Total  int64
 	Free   int64
 	FSType string