Usage statistics

This branch makes two changes which will be needed by the impending [ImageJ usage statistics project](https://github.com/imagej/imagej-usage):

1. It adds an `boolean official` flag to `UpdateSite` with corresponding API, so that update sites gleaned from the [official list of update sites](http://imagej.net/List_of_update_sites) can be flagged as such.

2. It adds a `getUpdateSite(File)` method to the `UpdateService` to identify which update site (if any) yielded a particular file of the ImageJ installation.

I looked into adding an automated test for the new `UpdateService` method but it would be quite involved, due to the updater's current architecture. I have been using this branch when testing `imagej-usage`, and the code is working.