Crawl multiple sources for the latest package version, determine release

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Taking Google Closure Compiler as an example, Maven has not picked up a release, but the tag is there on Github.

This is a bit of a challenge since different projects have different strategies for putting out releases, and there are multiple mechanisms for tracking releases.

But if implemented correctly, it will make versioneye much better for the users. We just track the projects we want to see updates for, and we get notified of updates without putting much thought into how the backend traces through multiple systems for releases.