lsjson: add IsBucket field for bucket based remote listing of the root

cmd/lsjson/lsjson.go

@@ -42,13 +42,16 @@ The output is an array of Items, where each Item looks like this
       },
       "ID": "y2djkhiujf83u33",
       "OrigID": "UYOJVTUW00Q1RzTDA",
+      "IsBucket" : false,
       "IsDir" : false,
       "MimeType" : "application/octet-stream",
       "ModTime" : "2017-05-31T16:15:57.034468261+01:00",
       "Name" : "file.txt",
       "Encrypted" : "v0qpsdq8anpci8n929v3uu9338",
+      "EncryptedPath" : "kja9098349023498/v0qpsdq8anpci8n929v3uu9338",
       "Path" : "full/path/goes/here/file.txt",
-      "Size" : 6
+      "Size" : 6,
+      "Tier" : "hot",
    }
 
 If --hash is not specified the Hashes property won't be emitted.
@@ -66,6 +69,10 @@ If "remote:path" contains the file "subfolder/file.txt", the Path for "file.txt"
 will be "subfolder/file.txt", not "remote:path/subfolder/file.txt".
 When used without --recursive the Path will always be the same as Name.
 
+If the directory is a bucket in a bucket based backend, then
+"IsBucket" will be set to true. This key won't be present unless it is
+"true".
+
 The time is in RFC3339 format with up to nanosecond precision.  The
 number of decimal digits in the seconds will depend on the precision
 that the remote can hold the times, so if times are accurate to the