Allow users to login without two-factor authentication

This version allows users to login without two-factor authentication.
If the username is not in the authy config file
then the user is able to logon using whatever strategy, such as pam or
certificates and we do not validate the second factor.
Fixed typos and removed trailing spaces.
Merge remote-tracking branch 'origin/client_3723' into new-release
Allow users to login without 2fa and removed pam dependency.

OpenVPN admins can now choose if they want to make 2fa mandatory. If
they're using pam, they can choose which users use 2fa and which don't
simply by keeping their logins out of the authy-vpn.conf file.

PAM is also no longer compiled as part of authy-openvpn as it's included
as part of openvpn anyway.
Removed .deb and .rpm packages generation.

We have removed the documentation and directories concerning to binaries
generation.
Upgrade to version 5.0.