Center Images in Getting Started guide, Adds Note Class to Getting Started CSS.

This Pull Request adds the "Center-block" class from Bootstrap to the images to put them in the center. It also adds the Note class to color the disclaimer in the Buttons on the bottom grey.

Example of Centered Image:
Before: http://puu.sh/8pvRR.png
After: http://puu.sh/8pvSm.png

Grey Disclaimer:
Before: http://puu.sh/8pvWm.png
After: http://puu.sh/8pvVB.png