Update download instructions

[Installation](http://docs.ghost.org/installation/mac/) and quickstart instructions currently prompt users to log in to ghost.org and then click a download button to download Ghost. There are two problems with this:

- Registration isn’t required to access Ghost's [download page](https://ghost.org/download)
- The “Download Ghost Source Code” button isn't visible to Ghost Pro subscribers. Here’s what a new, logged-in user sees on the Ghost homepage, where the “Get The Code” button links to the Ghost download page:

![screenshot on 2014-05-15 at 10-58-10](https://cloud.githubusercontent.com/assets/647669/2982625/a00ed462-dc17-11e3-89e6-248e89900cd0.png)

But here’s what a logged-in Ghost Pro subscriber sees on the homepage. It’s a list of their lovely blogs – there’s no download button, so the download instructions don't make sense when they say “click the blue ‘Download Ghost Source Code’ button”. (Theme and app developers are one group who are likely to want to run Ghost Pro and download the source, but there may be others too.)

![screenshot on 2014-05-15 at 11-00-53](https://cloud.githubusercontent.com/assets/647669/2982663/e9ea08e0-dc17-11e3-894d-9be14a1d97ca.png)

So I’ve updated the installation and quickstart instructions for all platforms and languages to:

- Omit mention of logging in to ghost.org first.
- Link directly to the download page at https://ghost.org/download/ instead of leaving users to find that page themselves.

I appreciate that asking users to register was perhaps designed to build Ghost’s mailing list, but since the download page is public and registration isn't required to use it, it feels like an unnecessary step.