Docs: Improvement to download page.

@grkvlt and I have tried to simplify the download page, while improving the ux for both newbies and seasoned pros.

We've introduced a table of downloads, including PGP and SHA1 links, and then simplified the subsequent paragraphs by removing content that would be duplicated.

The release/snapshot behaviour has been preserved.


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**Snapshot version** (Sonatype links):

![Screen shot 2013-04-05 at 16 13 05](https://f.cloud.github.com/assets/879000/344546/fb921aea-9e03-11e2-951d-bcbae64a2ee8.png)

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**Release version** (Maven links):
* Updated Maven all-search url
* All Jar includes link to current version link to useful Maven page  e.g. http://search.maven.org/#artifactdetails|io.brooklyn|brooklyn-all|0.5.0-rc.1|jar .

![Screen shot 2013-04-05 at 16 14 32](https://f.cloud.github.com/assets/879000/344547/fd92c7ae-9e03-11e2-8916-5df71fcb1b7c.png)