optimize images

run images through [ImageOptim](https://imageoptim.com/) to reduce file size for better page speed

![gulp-optimize-images](https://cloud.githubusercontent.com/assets/5244986/5569611/fb278d70-8f73-11e4-97da-522f7bebea34.png)

![gulp-imageoptim](https://cloud.githubusercontent.com/assets/5244986/5569612/fb2aa438-8f73-11e4-925f-a140240120e4.png)