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
Don't use github username

The username was never used for anything important. This also deletes
the whoami command.
Use default exercises dir for demo
Simplify config
Refactor config for clarity
Move normalization into commands
Bail if normalization fails
Move normalization code out of config
Start rewriting config
Move write method onto config pointer
Add new config command
Add comments about deprecating login/logout
Add home command. Fixes #95
Use go get on README when getting the project

Use `go get` instead of `git clone` for getting started with the project. This follows a bit more of the go convention and it places the project under the correct hierarchy under `$GOPATH`
Merge pull request #98 from Tonkpils/patch-1

Use go get on README when getting the project
Don't overwrite path if blank