BUG: .ix with PeriodIndex is fixed
Solves https://github.com/pydata/pandas/issues/4125

It doesn't feel like an elegant solution, but I think it works. 
I think when Period gets its own dtype, these will all go away?

pls add a whatsnew note
 merged via https://github.com/pydata/pandas/commit/879f5e90bea3b8c4849fd9f83a834e858dccd407

thanks!

(I changed this slightly in another commit). `is_array_period_like` does inference on the object and thus could be slow potentially
 Cheers @jreback!
