Fix link from "Getting Started" to "downloads" page

Fixes https://crosswalk-project.org/jira/browse/XWALK-878
Merge pull request #38 from townxelliot/download-link-fix_xwalk-878

Fix link from "Getting Started" to "downloads" page
Clean up indentation and remove commented-out code

This is preparatory to replacing the http client code.

Note that this changes the script's behaviour so that it no
longer shells out to the 'php' command: instead, we use a
client which doesn't need allow_url_fopen to be set to true
in the first place.
Make function to wrap URL-opening fopen() calls

This is preparatory to rewriting the code to open URLs using
something other than fopen.
Add an HttpClient class

This gives the benefit of being able to read the proxy.config
file in once, rather than for every request.
Handle missing proxy.config error properly
Switch to cURL instead of fopen
First pass at caching wiki page fetches
Add caching based on file mtime (5 minutes)
Provide an error message if directory is not writable
Ensure missing() is given a file name
Use print() instead of echo() for consistency
Write HTML files out for 'documentation' and 'contribute'

This is necessary for when the gfm.php script is being used
to generate the live static site.
Add note that cURL extension for PHP should be enabled
Split wiki history generator out into separate file

This is to enable it to be called from regen.php without having
to shell out to php.
Extract wiki/pages.md.html generator into separate file

This is to enable the regen.php file to invoke the same functions
without needing to shell out to php.
Fix error where variable was not found

I removed the $path variable (which is just distracting), and
neglected to remove references to it.
Reinstate path variable

Just in case the wiki moves somewhere else at some point.
regen.php no longer needs to shell out to php
Add note about how to setup the github service hook

The service hook for changes to the wiki has to be configured manually
via the github API. This is because the standard web hook configurable
through settings for the crosswalk-website project does not send
notifications for changes to the wiki.