Implemented 'ls' command;
Modified unit tests to suppress output deprecation warnings from paramiko;
Modified unit tests to perform authentication in setUp method;
Added 'ls' unit tests;
Closes #20
[x] The directory tree of the remote server should be available to the user
[x] All directories should be included in the correct hierarchy.
Implemented 'ls' command; Modified unit tests to suppress output deprecation warnings from paramiko; Modified unit tests to perform authentication in setUp method; Added 'ls' unit tests; Closes #20