Create a public client strategy from client password

Alter jaredhanson/passport-oauth2-client-password to create an
"authentication" strategy for clients which merely identify themselves.

In particular, [Section 4.1.3][oauth] of the OAuth2 spec allows for
clients which have not been issued passwords to exchange authorization
codes for access tokens, provided they send their client_id so they
do not inadvertently receive an access token intended for another
client application.

[oauth]: http://tools.ietf.org/html/rfc6749#section-4.1.3