fix(card): made md-card themeable

Hello There!

I'm really excited by what you have done so far with Angular Material Design. I've been working on building a POC for a 2015 project using angular and ngMaterial and I noticed that themes include css for md-card but md-card isn't themeable. This PR makes the md-card directive themable and adds two karma tests to confirm that it is themeable. Apologies in advance if I missed something or if md-card is intentionally not themeable, OR if I did not submit this issue/PR in the proper way.

Suggestions, feedback and schoolings in general always welcome as well! 

Thank you,

-Marc