API: consider better decorator option
It's not an issue in py3.5, but in older versions this is annoying - because a decorator is used in the definition, the function signature on `to_datetime` is lost.  This is the big one I've run into, but there may be some other places in the API it comes up.
![image](https://cloud.githubusercontent.com/assets/1924092/17310153/49ca6d18-5808-11e6-91aa-483ab3458efd.png)

I saw this blog today - could use one of the strategies there.  It may make sense to vendor rather than adding another dependency?
https://hynek.me/articles/decorators/
