Implemented 'chmod' command; refactored unit tests

[eddiekelleypdx]

Implemented 'chmod' command; Refactored unit tests to provide better structure of TestCase classes; Closes #26

• [x] After changing permissions on a file, that file will end up with the specified permissions
• [x] Users with read/write access to the file should be able to change its permissions
• [x] Users with read-only access to a file should not be able to change its permissions
• [x] File must exist for the user to be able to change permissions on it