add credentials and oauth helper request procedure

* oauth/oauth1/signature.scm: define credentials. Credentials contain a
  consumer key and secret and also the signature algorithm to be used
  for signing the requests.

* oauth/oauth1/client.scm: use credentials and define new
  oauth1-client-request helper function.

* examples/oauth1/twitter.scm: use credentials and new
  oauth1-client-request helper function.