AuthorizationShell and Example changes

- Added additional Keychain instructions in AuthorizationShell note.
- AuthorizationShell: Fixed bug where UserCancelled error was not being
properly handled.
- AuthorizationShell: Added Reset method.
- AuthorizationShell: Removed unneeded buflen property.
- AuthorizationShell Example: Added Username and Password field with
code.
Minor edits
Updated release notes

Version 171.
Merge pull request #137 from ktekinay/master

AuthorizationShell changes
Merge pull request #63 from macoslib/master

Merge changes