Create custom google now cards
There is currently no way to do that. Google makes its own cards and custom application cannot register any cards. But I hope it will be possible in future.
Actually Google announced last week that developers can now develop custom Google Now cards:
http://www.google.com/landing/now/integrations.html
However, a developer guide seems not available yet.
Edit: On the end of the page they point out that:
We'll let you know when we are able to onboard more partners