CertificatesFragment base functionality added

1) fragment_certificates.xml added - contains the list view that holds
the certificates
2) CertificatesPersister added - manages the http requests for the
certificates
3) list_row_certificate added - the UI of a single row of the list view
4) CertificateAdapter modified - manages the certificates list view
5) CertificatesFragment implemented - there is still work to do though