Use Provider's default `identifier` if no "id" param given.

Upon init of OpenID provider objects, the predefined `identifier` value
that may be present in specific provider classes like Yahoo and Google
was being clobbered by the parameter lookup if no "id" (or similar)
parameter was provided.

Also modified flow logic to use the provider object's `identifier` field
instead of always requiring an "id" (or similar) request parameter.
Merge pull request #25 from jmurty/feature/use-default-provider-identifier

Use Provider's default `identifier` if no "id" param given.