Update acknowledgments to recognize GitHub

Signed-off-by: George K. Thiruvathukal <gkt@cs.luc.edu>
Update download links to go to GitHub instead of Bitbucket

Signed-off-by: George K. Thiruvathukal <gkt@cs.luc.edu>
Update additional references to go to GitHub instead of Bitbucket.

Signed-off-by: George K. Thiruvathukal <gkt@cs.luc.edu>
Merge branch 'master' of github.com:LoyolaChicagoBooks/introcs-csharp
Merge pull request #3 from gkthiruvathukal/master

Updates associated with Bitbucket -> GitHub migration