Switch readme badges to svg

SVG readme badges are crisper :)

Before:
![screen shot 2014-11-25 at 14 38 43](https://cloud.githubusercontent.com/assets/162976/5177687/86a13762-74b1-11e4-9501-029a5a72e73e.png)

After:
![screen shot 2014-11-25 at 14 44 48](https://cloud.githubusercontent.com/assets/162976/5177693/a2562882-74b1-11e4-838f-f5f5cbd7ff7f.png)