Deprecate Downloads API

The [Downloads API](http://developer.github.com/v3/repos/downloads/) has been officially deprecated since we said "[Goodbye, Uploads](https://github.com/blog/1302-goodbye-uploads)".

This adds documentation for that deprecation.

An alert has been added to the Downloads API page:

![screen shot 2013-12-05 at 16 54 30](https://f.cloud.github.com/assets/65057/1684030/b58c9e6c-5dc5-11e3-8d65-ed5f88c931b8.png)

And the appropriate deprecations have been added to the Deprecations list:

![screen shot 2013-12-05 at 16 55 21](https://f.cloud.github.com/assets/65057/1684034/bbe4c122-5dc5-11e3-967f-df83980245e3.png)

Related to this, today I also updated the [blog post](https://github.com/blog/1302-goodbye-uploads) announcing this deprecation, to change the originally planned "disabled in 90 days" to "disabled at a future date".

![screen shot 2013-12-05 at 16 58 10](https://f.cloud.github.com/assets/65057/1684064/156c854a-5dc6-11e3-84b5-cbd0f5a23b54.png)