Generates new token from old token.

The API Specification http://api.openstack.org/api-ref-identity.html allows users to authenticate and generate token from old token.
Request body format:
    {
        "auth" :
            { "token" : {"id" : "old_token"} }
    }

such request:
    1. validates the token
    2. decodes the token
    3. and generates new
Username/Credentials retrived from token driver

Since each token driver is free to use the way it wants to store the
username/password, username() and credential() method are invoked to get
the values. token_driver.credentials() added in view of this need.
Removed _is_token_auth()

If request contains token id it is assumed to be token auth.
Merge pull request #59 from amol/token-using-token

Generates new token from old token.