Prompt to move Pester to the Applications folder.

Code from https://github.com/potionfactory/LetsMove/, with modifications:

- Check for disk images properly.  Don't just guess.

- Use absolute paths everywhere in NSTask.

- Escape quoted paths.

- Try to delete rather than trash to avoid upsetting Hazel.

- Change some strings to improve clarity.

- Remove pre-10.6 support.