Sort by downloads without letter restriction

* 'Browse All Crates' links to all crates instead of all
  crates that start with the letter A.
* Changing the 'Sort by' param returns the user to the first page.
* Crates pagination only shows the surrounding 9 pages instead
  of all possible pages.

Closes #44.
I think that this should probably be done for all pagination, so you may just want to merge the `surroundingPages` method into `pages`. If we need to we can add an `allPages` method later on.