Added basic HTTP auth for /change_hook/github

For testing append this line to master.cfg

> c['status'].append(html.WebStatus(http_port=8011,allowForce=True,change_hook_dialects={'github' : True},change_hook_auth=('user', 'password')))

start master and open http://localhost:8011/change_hook/github then auth window should appear.

I didn't test it for /change_hook/base but I guess it will works too