Add authenticating onto client site

[jwidavid]

This is designed to be one step toward completing the MVP feature here: #253

Changes:

• Uses the authenticated cookies in a subsequent scan
• Adds unit tests and error handling

Testing instructions can be found in the above PR as well.

Additional testing

• Try causing an uncaught exception.
  • Manually close the popup browser window before continuing
  • Don't authenticate but press enter in the terminal to continue

[coveralls]

coverage: 98.84% (-0.2%) from 99.045%
when pulling 77ab7f0 on add/saml-cookie-management
into 6ad3fbf on add/saml-authentication.

[miguelxpn]

LGTM! Works as advertised too!