Performs a simple backup on a specified directory.

The files it backs up are determined by the following rules:

• It recursively traverses the directory specified looking for files that should be backed up.
• If a .gitignore file is found then the files and driectories specified to be ignored will not be backed up.
• .exe files will not be backed up.
• Files are only backed up if they are newer then the ones in the backup.

• It is very slow. Perhaps it can be speeded up by:
  • Writing it in an asynchronise style.
  • The ignoring of files in the .gitignore file is currently performed by starting a process and running git check-ignore. Parsing the .gitignore file directly (using, for instance, the crate ignore) could be quicker.
• If a .rackup_ignore file is found then the files and directories specified in it will not be backed up.
• Have the backup directory specified by an environment variable.