Update download main page

[kcrisman]

Just as the wiki page for supported platforms was really quite out of date, now it also is the case that the mirror download pages are quite out of date - with mentions of Solaris, ova, livecd, and various other things that, while not worth ditching, should be strongly de-emphasized since they are no longer providing current versions of Sage.

See also #186, #131, #174, and even #193 to some extent for other issues of rotting on download pages for the website. Unfortunately, it's not always easy to find the exact place some of the mirror template files live in order to update them.

[haraldschilly]

well, this sounds like we should tune the main index page a little bit. e.g. the main choices at the top and everything else down below as an extra? the content right now is this:

 <h1>Sage Mirror Content</h1>

 <ul>
  <li><a href="./win/index.html"><strong>Microsoft Windows</strong></a> — Install SageMath on Windows</li>

  <li><a href="./ova/index.html"><strong>OVA Image</strong></a> &mdash; Image for virtualization, also a binary distribution for Windows</li>

  <li><a href="./livecd/index.html"><strong>Live CD</strong></a> &mdash; Bootable Sage Live CD</li>

  <li><a href="./linux/index.html"><strong>Linux</strong></a> — binaries, for 32 and 64 bit</li>

  <li><a href="./osx/index.html">Apple <strong>Mac OS X</strong></a> — binaries for Intel and PowerPC systems</li>

  <li><a href="./solaris/index.html"><strong>Oracle Solaris</strong></a> — binary and source</li>

  <li><a href="./src/index.html"><strong>Source Code</strong></a> —
     source code of Sage
     (<a href="http://old.files.sagemath.org/src-old/">older versions</a>)
  </li>

<li><a href="./spkg/"><strong>SPKG Sage Packages</strong></a>  &mdash; local mirror of optional and experimental Sage SPKGs.</li>

  <li><a href="./devel"><strong>Development</strong></a> — Source code of latest development release</li>

  <li><a href="./doc/"><strong>(Local) PDF Documentation</strong></a> — local copy of Sage's documentation.</li>
</ul>

<h2>Additional Information</h2>

<ul>
  <li><a href="./torrents.html"><strong>BitTorrents</strong> for all Downloads</a> &mdash; <a href="http://www.bittorrent.org/">BitTorrent</a> is used for distributing large amounts of data over the Internet in a P2P network.</li>

<li><a href="http://maps.google.com/maps/ms?ie=UTF8&hl=en&msa=0&msid=106570724428886886399.00046d7f23e304afe9f11&z=2">Other <strong>Mirrors</strong> Worldwide</a> &mdash; in case you need another mirror select one here</li>
</ul>