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.
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.
Invalidate the cache for wiki pages changed on github

The regen.php script will get called by the hook each time a
gollum (wiki) event occurs on the crosswalk-website project wiki.

We now loop through the changed pages notified in the event, and
remove the cached HTML for any which have changed.
Fix PHP cURL instructions for Linux

*.dll is for Windows, should be *.so.
Choose an HTTP GET approach based on capabilities

Rather than expect a particular implementation to be available,
iterate through the possible approaches to fetching URLs until
we find one which works.

Preference is given to cURL, followed by fopen (if allowed by
the PHP configuration), then shelling out to php cli which invokes
fopen (allow_fopen_url doesn't matter for php cli).

This is quite nasty, but I didn't want to remove the cURL
implementation as I'd like to be able to migrate to this if we
can get the server configuration changed.
Fix arguments to fopen

The stream context needs to be the 4th argument; the 3rd argument
is use_include_path, which we can set to false, as we are always
fetching from remote locations with HttpClient.
Fix typos
escapeshellarg is not suitable when fetching URLs

Using escapeshellarg on a filename before passing it to the HTTP
client doesn't work, as it produces URLs like this:

http://localhost:4567/'documentation/00-Getting-Started.md'

which are invalid.