Be more intelligent about retrieving release data

[techman83]

Currently we scan the whole lot, and update anything that changed.

We can use the latest for github and use:

curl https://api.github.com/repos/SirDiazo/ActsEverywhere/releases/latest
"published_at": "2015-05-10T04:05:28Z"

The kerbel stuff api doesn't appear to have a last updated, but we could use the 'Last-Modified' header of the latest download.

Jenkins may update meta data on PRs, but need to check that. We can also register some github hooks for NetKAN metadata updates easy enough.

'If-Modified-Since' Could also be used if the remote supports it in the options request.

Probably worth breaking out a full re-index option, webhook and regular checking.