Fixed* Issue #4

Hello, I don't know if I'm being useful or annoying, but I've forked PyLaTeX and added functionality described in Issue #4. I see this project as something I have use for, and as a CS student at university, I need a strong Open Source GitHub portfolio.

If you'd like me to continue contributing, I'd be happy to.

*![pylatex-multirow](https://f.cloud.github.com/assets/1160189/2003014/a25af18e-8626-11e3-854c-e70d47200d02.png)