Allow providing default a SecurityContext object during initialization

[chrbayer]

Such a customize SecurityContext object allows the providing certificates for the https communication and thus allows the usage of a server which uses self-signed certificates.

This fixes issue #71.

    ByteData certificateAsBytes = await rootBundle.load('assets/example-ca.crt');
    List<int> certificates = certificateAsBytes.buffer.asUint8List();
    final SecurityContext customSecurityContext =
        SecurityContext(withTrustedRoots: true)
          ..setTrustedCertificatesBytes(certificates);
    Parse().initialize('example-org-parse', 'https://example.org/parse',
        securityContext: customSecurityContext);