update default exercism assignment path to ~/exercism

Previously the default path (if none was provided by the user) when logging in defaulted to a folder called exercism in the current working directory. Now the default path is relative to the user's home directory. If the user requests a demo the demo directory will also be relative to the home directory.

Feedback welcome!

Closes #96
This should be an `errors.New()` rather than a panic, and we should handle it in from `main()` my informing the user and doing an `os.Exit(1)`.