panic if we are unable to determine where home is

Returning a blank string if the user has an incorrectly setup
environment could mean we puts things in unexpected places.
have demo run from ~/exercism-demo

There is no longer any errors to return, so I've remove the error from
the method signature.
add homeDir to make testing easier

This allows us to set a home directory when testing, which makes it
easier to work with tests that depend on its location.
change default exercism path when logging in

* use config.DefaultAssignmentPath when user input is blank for
directory
Merge pull request #97 from lcowell/default_exercism_directory

update default exercism assignment path to ~/exercism