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.
Would it be possible to refactor this such that the number of pages is a constant listed in one location? In theory all the other numbers would be based off that one constant.