BUG: allow conversion of Timestamp and Timedelta to string in astype
Fixes GH #9757

this needs some discussion
pls post in the issue why this should actually be allowed
this is string formatting and not the meaning of astype 
 ok, seems reasonable. can you rebase / squash. ping on green.
 Added a test for `astype(unicode)` on Python 2, and a test for DataFrame.
 OK, this seems good to from my perspective. I'll merge unless there are any further objections....
 I've totally borked this branch with an accidental force push. I'll fix it tonight. On the positive side, I finally learned what push.default=simple does :).
 Should be fixed.
 Thanks @evanpw
