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. 
Thanks for taking a look! I implemented that in https://github.com/rakyll/drive/pull/24. Are you saying there is some additional work?