Horizontal section on overview page for link to Try Git

This implements the changes discussed in #80—i.e. removing the Try Git callout from the hero block and giving it a full-width horizontal section for it further down on the page. It’s been added as a partial too, in case we ever want to include it elsewhere on the site :grinning: 

:zap: Props to @dandenney for proposing the change!

Here’s what it looks like:
![Screen Shot 2013-03-08 at 10 47 18 AM](https://f.cloud.github.com/assets/296432/236216/965b1b18-87dd-11e2-98da-bc0db39204b2.png)

It also collapses down where possible on smaller viewports. An example:
![Screen Shot 2013-03-08 at 10 45 44 AM](https://f.cloud.github.com/assets/296432/236215/577ba976-87dd-11e2-94c3-d9c91c21b961.png)

Any more suggestions or concerns before this gets merged?

/cc @github/training-team @Gregg