Fixing Provider initialization in Authomatic.access(), fixes #23
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.
Ensure deserialization of session data always returns non-None object.

When deserializing invalid session data, such as an expired session,
the `_get_data()` method sometimes returns None. This fix ensures that
the `data` property always returns an empty dict instead of None in this
and all cases, to avoid breaking subsequent get lookups of the session
data.
Merge pull request #24 from yprez/fix_access

Fixing Provider initialization in Authomatic.access(), fixes #23
Merge pull request #25 from jmurty/feature/use-default-provider-identifier

Use Provider's default `identifier` if no "id" param given.
Merge pull request #26 from jmurty/feature/fix-session-data-to-never-be-none

Ensure deserialization of session data always returns non-None object.