[SYSTEMML-1256] Add download link for previous releases
[SYSTEMML-1256] Add download link for previous releases

Added reference to previous releases at
http://archive.apache.org/dist/incubator/systemml/ for download page.

Closes #39.

gweidner/systemml-website,apache/incubator-systemml-website,apache/incubator-systemml-website,objectadjective/incubator-systemml-website,objectadjective/incubator-systemml-website,gweidner/systemml-website