support JSON Web Token

[shogo82148]

Hello
I want to use Service Account for authorization but Net::Google::DataAPI seems not to support it now.
So, I implemented Using OAuth 2.0 for Server to Server Applications.

Net::Google::DataAPI::Auth::OAuth2::SignedJWT uses private key instead of password for authorization.

use Net::Google::DataAPI::Auth::OAuth2::SignedJWT;
my $oauth2 = Net::Google::DataAPI::Auth::OAuth2->new(
     private_key => <<'__KEY__',
-----BEGIN PRIVATE KEY-----
xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
-----END PRIVATE KEY-----
__KEY__
     service_account => 'xxxxxxx@developer.gserviceaccount.com',
     scope => ['http://spreadsheets.google.com/feeds/'],
 );
$oauth2->get_access_token or die;
my $client = Net::Google::Spreadsheets->new(auth => $oauth2);