Add methods to list files in a RemoteData object

[giovannipizzi]

Things like remotedata.listdir() that will ssh (if needed, it should use the transport) and list the files.

I will also add remotedata.listdir_withattributes() both the RemoteData class and directly to the Transport class, to get not only the filename but also some metadata (at least if it's a directory or not, to be able to recursively list a directory).

Also to add:

• tests
• command line command verdi data remote ls
• command line command verdi data remote cat

[giovannipizzi]

Closed with #743