Better Exceptions

Pull request for the previously added (and in theory approved) issue #160.

- All errors are now defined under the YouTubeIt namespace and inherit from YouTubeIt::Error
- All 404 responses from the YouTube api now raise YouTubeIt::ResourceNotFoundError instead of UploadError

Note that some tests for the handling of errors have been added, making the code more robust overall.

Also adds a change log since it's crucial to inform people using the gem of what's being changed between versions. Whenever you actually release a new gem version make sure to set the appropriate version number and date info in the changelog as well.