Currently, review ids that are rounded numbers or palindromes get a special trophy in the review detail page. Added Prime numbers to that list. 

Summary of Changes:
* reviewboard/reviews/templatetags/reviewtags.py
  + Added a isprime method that takes an integer and figures if its a prime. (Logic Taken from Stackoverflow and modified to ensure 1 is not a prime number) 
  + Make isneatnumber check for prime numbers. The restriction for ignoring ids less than 1000 has been relaxed for prime numbers. (Not sure if you would want this logic for primes)

* reviewboard/reviews/tests.py
  + Added tests for prime numbers
  + Modified one of the ids for the testPalindrome as that happens to be a prime as well

* reviewboard/templates/reviews/trophy_box.html
  + I dint know what image to use , so the patch currently uses the same "trophy.png" for both milestones and primes. This would need to be changed.

Ran the test suite