FIX DataFrame diff with timedelta 
fixes #4533

_need to fix test in numpy 1.6..._

hmm seems a bit special casey
 true, perhaps better to change the dtype of arr_out / view, but couldn't get that to work...
 I think you need to set the dtype of `out_arr` in the if block (e.g. if its float/int/datetime64/timedelta64) as it needs to be `i8` (then you take a view before you return it, so need to remember to do that)

e.g. `return out_arr.view('timedelta64[ns]')`
 @hayd ?
 @hayd ?
 @hayd ?
 @jreback Did you think this was too special casey? (will change the view dtype and see if this passes).
 @hayd it might be, but not sure how can avoid this, `timedelta` is ITSELF special casey (integer and datelike at the same time)
 @jreback ok cool, in general it's good to be uncomfortable/point out/whine about special casey-ness. :)
 closing this as picking it up in #8184 
 thanks @jreback, sorry for dropping the ball on a few PRs.
