[KeyManager] getCertificate and saveCertificate

[KeyManager] getCertificate and saveCertificate

[Verification]
var cert = new tizen.Certificate("cert3", null, true);
cert.save("MIIDGDC...DVC8w==",
    function() {
        console.log('ok');
        var c = tizen.keymanager.getCertificate("cert3");
        console.log(c.name);
        console.log(c.password);
        console.log(c.extractable);
        console.log(c.rawCert);
    },
    function(e) {
        console.log(e.code);
        console.log(e.message);
    }
);

Change-Id: I44ff150d6e83a9715d86e0bc1ca659047bc40a9c
Signed-off-by: Przemyslaw Ciezkowski <p.ciezkowski@samsung.com>