Certificate formatting and retrieval

This change introduces the implementation of the SSLConnection methods
getpeercert and cipher. The following has been added:

    * dtls.util:      utility elements shared by other modules in this package
    * dtls.x509:      a module for X509-certificate-related functionality,
                      including formatting a certificate into a Python
                      dictionary as prescribed by the Python standard
                      library's ssl module; functionality for testing with
                      PEM-encoded certificates in the file system is included
    * yahoo-cert.pem: the current certificate of www.yahoo.com: this is a good
                      testing certificate, since it contains the subject
                      alternate name extension

Other notable changes:

    * sslconnection:  private attributes are now preceded by "_"
    * openssl:        null-ness in opaque FuncParam-derived return values is
                      now properly detected and an exception is raised as
                      expected

dtls/test/certs/yahoo-cert.pem

@@ -0,0 +1,29 @@
+-----BEGIN CERTIFICATE-----
+MIIE6jCCBFOgAwIBAgIDEIGKMA0GCSqGSIb3DQEBBQUAME4xCzAJBgNVBAYTAlVT
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+inu9YTrN/4P/w6KzHho=
+-----END CERTIFICATE-----