Implement a list method

This change implements a barebones list method to let users see what files exist on their
drive (a nice complement to the pull function).

Right now it just dumps a list of files and directories:
If the path is a file then it is listed.
If the path is a directory then the direct children are listed.

Note this partially address issue #9. 

Future work. Add options to list details about files in the cloud. Make it so users can tell files and directories apart. 
We may need to alter what we print out depending on the file MIME type, need to distinguish directories from regular files.