BUG: start_time end_time to_timestamp bugs #2124 #2125 #1764
@wesm can you review this plz?

The main change is I have to_timestamp default to second frequency now to get the first and last second of the Period. This doesn't **quite** solve #2125 since Timestamp resolution goes down to Nanos. So the question is is it appropriate to add 999999999ns to the end_time when ns doesn't exist asa period freq? 
