BUG: preserve frequency across Timestamp addition/subtraction (#4547)

My second PR, on basically the same code.

Assuming people agree with my comments on #4547 (that users adding timedeltas that don't match frequencies better know what they are doing), then this would be my submission.

I'm also assuming that something is up with Travis and it is not anything I have done - nosetests passes fine for me though Travis still dies more or less at startup, while it didn't on older code. That problem is true of builds not including any code I have added, e.g. https://travis-ci.org/pydata/pandas/jobs/20167915 . Maybe something in PR #6068, or does this kind of thing happen from time to time?