Adding Integer filter

@richsage I've tested this against Ethex.  However, Ethex uses Symfony 2.1 (and thus an older AdminBundle), so I don't know whether additional testing will be needed as master here is on 2.3.  Let me know your thoughts.

Also note that I'd like to back-port this fix into the 2.* branches once it's merged into master.  Would you suggest that I do that locally and push up, do additional Pull Requests or something else?