Add CTAs to install extension/apps

This pull request adds a page at `/install` which uses the same template as the homepage, except adds an install banner at the top:
![screen shot 2014-07-08 at 14 11 53](https://cloud.githubusercontent.com/assets/84700/3514480/60cd064a-06cc-11e4-9cfb-fd94a35c425c.png)

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The background of the banner has some transparency, and on large screens the banner is fixed:
![screen shot 2014-07-08 at 14 12 47](https://cloud.githubusercontent.com/assets/84700/3514502/8bd52534-06cc-11e4-8bac-052c6a8fdb85.png)

***

On smaller screens, the buttons are stacked:
<img src="https://cloud.githubusercontent.com/assets/84700/3514484/64ea4e72-06cc-11e4-95e9-0918c47b0840.png" alt="screen shot 2014-07-08 at 14 13 02" width="248" height="508">

***

The header suffers a little on mobile due to this new banner, but that's only a consequence of the existing mobile experience not being that great.

Given that this page is geared toward installing specifically, I think the compromise is OK.

I improvised this whole thing, so feedback is encouraged.

cc @bergie @d4tocchini
Not sure if we want to wait until we have URLS for these before merging.