Issue 9: Completed refactoring so that the work for #3 can be carried out

* ```AbstractGithubNameScreen``` class was created. This will allow an extension in #3 for a user to be able to modify their github account name if necessary.

* LoginPage changed to EntryPage since it is the point of entry - just to give it a more meaningful name.

* Return type of ```registerKerberosUser``` on ```LoginService``` changed to a void return type.

* Dealt with bugs related to GWT conventions (the onSuccess() and onFailure() calls). Exceptions should only be thrown if and only if an operation was incomplete.

* Main page flow changed. The form to register a new github username within the server context was moved to it's own page.