updating background sass property list

When writing SASS I'm having the problem that the "background-size" is not being highlighted.
I reassured that this problem does not lay in a broken theme. 

See this screenshot:
![screenshot 2014-10-27 10 22 07](https://cloud.githubusercontent.com/assets/2418899/4793547/bc0a6b96-5dee-11e4-9748-6608c5eb8f7b.png)

Browsing through the grammar, I found that "background-size" was not listed. So I went ahead and added it. 
This is my first contribution to a project so I hope I'm not getting it all wrong in here.

This pull request is meant to make the background-size attribute be correctly highlighted.

Please advice if I'm wrong.