Browse files in sharing directory

We need a new class Util in module util.py in order to host a ner function that will browse all shared files in ShellSettings.DIRECTORY

This function must return a list of dictionaries.

Each dict is composed in this way:

{
  "filename": "bla_bla_bla.mp3",
  "dimension": 3,
  "sha1": "...."
}

• filename is just the name of current file
• dimension must be showed in MB
• sha1 is sha1of the current file

This list will be used in register call as part of its payload.