Add class for accessing github API

This uses the site-config.json file to store the credentials
to do authenticated requests to github; without this, it
still works, but does unauthenticated requests which are
rate-limited to 60 per hour.
Only use cURL in HttpClient

Strip out fallback code for command line and fopen, as we
now have cURL on the server; we also need to use the client
in ways which aren't naturally supported by fopen (e.g. reading
headers) and don't want to have to implement them three times.
Return associative array as response from get_url()

This enables more complex behaviour, such as checking for 304
(Not Modified) status codes, which we can use to control how
often we hit the github API etc.
Use PHP github proxy in channels viewer
Split JS utils into separate reusable file

This is to enable reuse of the Ajax client.
Add HTTP client which uses 'If-Modified' header
Add Cache class
Use If-Modified-Since header for github fetches
Clean up querystring generation for github requests)
Use github proxy for Downloads page
Extract JS for creating Crosswalk download URLs
Downloads page now uses github API
Fix some incorrect comments about CSS
Fix download table column widths

Set each column to 1/3 of the remaining width in the Downloads
table, so that columns don't reflow when the links are populated
by Ajax calls.
Not a good idea to put credentials in JSON file...

Noticed it before it went to live, though.
Update instructions for github API credentials
Log errors instead of failing when cache is unwritable

The HTTP response will still be returned, but will not be
cached.

This is to prevent the Downloads page falling over if the
cache directory cannot be written to.
Allow unauthenticated calls to github if site-config.php is missing

This is so that the site will continue to work if site-config.php
is temporarily unavailable or missing.
Tizen mobile canaries are now for Tizen 3.0
Add more instructions about permissions on staging/live