documentation and refactorings

* oauth/oauth1/credentials.scm: credentials defines a pair of unique
  identifier and a secret (used in client credentials or tokens).

* oauth/oauth1/client.scm: return tokens as credentials.

* oauth/oauth1/request.scm (oauth1-request-sign): new method for signing
  request. the signature module is only used to define signature
  algorithms.

* oauth/oauth1/signature.scm: request signature moved to request module.

* oauth/oauth1/utils.scm: added parsing of
  application/x-www-form-urlencoded content type strings.