WEB: Add a "Recommended download" button to the downloads page

The idea has been taken and adapted from the VideoLAN web site
(http://www.videolan.org). A big 'download' button is shown,
which links to the most appropriate version for the user, depending
on the OS reported by the user's browser. Only OSes with a browser
have been added. Known browser user-agent strings have been taken
from http://user-agent-string.info/list-of-ua/os
WEB: Bugfix for the default value of the recommended download
WEB: Cleanup download paths for the recommended download
WEB: Fix navigation links in the games page
WEB: Add a note about changing RewriteBase when debugging locally
WEB: Generate the list of recommended downloads from the XML data

Add a new XML field, "user_agent", used to find the user agent string
where the download item in question will be suggested to the user. Only
items with the "user_agent" field can be provided as recommended
downloads
WEB: Update the download backgrounds to better match our GUI colors
WEB: Fix download button background images again

We also need to take padding into account, which was omitted in the
previously updated images
WEB: Update the download button background colors to match the new images
WEB: Only show the recommended d/l button if a user agent match is found

Also, the creation of the button itself has been pushed to Javascript.
The recommended d/l section is now only shown if JS is enabled and an
appropriate user-agent has been matched
WEB: Specify the correct user-agent for Intel/PPC Macs and iPhone/iPad
WEB: Don't use undefined variables when the user-agent can't be matched
WEB: Remove recommended downloads for all *nix flavors

It's not possible/reliable to distinguish *nix flavors from the web
user-agent.
WEB: Add a recommended download button for Ubuntu distros again

It seems that the "Ubuntu" string is part of the user-agent for some
browser versions in Ubuntu, thus we could use it for our recommended
download button, if the string exists
Merge pull request #8 from bluegr/recommended_download

WEB: Add a "Recommended download" button to the downloads page