tweak unicode() for PeriodIndex to yield valid python code

_p=PeriodIndex([u'2000-01-03'],freq='B')
print unicode(_p)
PeriodIndex([u'2000-01-03'], dtype=int64)

can't recreate object with result from str(self).

[cpcloud]

do u want to keep the current repr? and is it ok to put eval calls in these tests? i'm going to guess u want to keep the repr behavior since one would not want to see a 1e6 length period index print out.

merged the PR, using eval is fine.