Add windows username to api and configuration

[richfitz]

Merge after #40, contains those commits

This PR adds a helper function windows_username() which returns the username, and then uses that function to add the username into the reported configuration from hipercow_configuration()

[weshinsley]

Looks good - additionally tested it by windows_authenticate() with both my "normal" and "admin" usernames, and in each case, correct user name was reported. (Not many people will have two valid windows usernames)