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 basically recreates the same logic as the implementation, which I don't think is a particularly useful test.

I'm OK with not testing this, or testing it by making some sort of assumption about the end of the path matching something. Or by explicitly setting home to `/tmp` and then asserting on the actual string value.

Overall operating with a package level variable will not be safe in terms of race conditions, so perhaps this should be a value on a config object itself, accessed through a method so that it can be cached or overwritten.