Fix revision in channels' JSON

Revision key in channels' JSON has been changed recently.

Also, harden the respective url by using HTTPS.
Improve string casting of revision info values

Some revision payloads contain git hashes in place of revision number
for built-in components (webkit, v8).
We should use universal -description instead of NSNumber's -stringValue.

See revision #278870 for example.
Enhance UX of revision downloading progress

Revisions are already spread across almost 30 XML files 1000 builds
each. Although it may take a considerable amount of time to download the
list, the progress bar doesn't reflect this process.

Because the downloading isn't deterministic, we try to update the
progress after each received file. 0.02 rate should be OK for user and
enough for around 2 years of Chromium builds before hitting bar's 90% limit.
Bump version
Merge pull request #3 from ardecvz/fix-revision

Fix revisions downloading. Thanks ardecvz
Updated to support 64bit experimental, new source url, and custom prefix.