Add failed jobs column to rqstats command

[dangquangdon]

Regarding #416

Add Failed column to rqstats management command. It would look like this

Django RQ CLI Dashboard

------------------------------------------------------------------------------------------
| Name           |    Queued |    Active |  Deferred |  Finished |    Failed |   Workers |
------------------------------------------------------------------------------------------
| default        |         0 |         0 |         0 |         2 |         0 |         0 |
------------------------------------------------------------------------------------------

[selwin]

Thanks!