Generate sitemap for metacpan.org website

This introduces a script called bin/generate_sitemap.pl that creates XML files
containing URLs for authors, releases and modules; a module that does all of
the heavy lifting; a test script that exercies the module; and an updated
robots.txt file.
Also add ElasticSearch
Use FindBin to get lib path
Limit release to latest ones only

Previous version dumped all releases; releases are now limited to just the
latest ones, bringing that file in line with the size of the others.
Create xml.gz files, not just xml files
TidyAll
Use autodie.
Sort urls in sitemap.
Convert MetaCPAN::Sitemap to Moose.
Sitemap release URLs now point at /release/ rather than a download URL.