v2.1.1

Bug fix and clean up

- Added requirements, installation and contributing information to
README.md.
- Added OAuth2::sessions() to check if sessions are enabled and one is
active.
- Fixed OAuth2::iloginURL(), OAuth2::iloginButton() and
OAuth2::iloginRedirect() from generating bad links and
outputting/logging undefined variable: $redirect_uri errors.
- OAuth2::loginRedirect() now checks if headers have already been sent.
- Added OAuth2::loginRedirect(string $message), to output a string
before stopping the script.
- Added license to composer.json.