Improve tests for the changes_since_last_scan field

This improves test coverage of the `changes_since_last_scan` field.

Implementing the test, I found a problem with the code.  `Page::getPreviousScan()` returned only the previous page scan within the current site scan.  For  `changes_since_last_scan` to be set correctly, we shouldn't care what site scan the previous page scan is in, as long as it is the previous scan on the page.

This means that the index of (pages.scans_id, pages.uri) will not help with the new functionality of `Page::getPreviousScan()`, because `pages.scans_id` is not being searched against.  While we still need that index, we now need a new index for just pages.uri.  Which brings us back to the problems of #29.  So this, pull request re-introduces the hashed uri proposed in #29 to improve performance. 

![fy75ivc](https://f.cloud.github.com/assets/498678/2232418/c053350c-9b1b-11e3-84ef-530f3c92bbbb.gif)