Add origin parameter to Timestamp/to_datetime epoch support.
When using SAS or Stata data, dates are represented as the number of days since 1/1/1960, and other statistical software uses different [origin dates](http://blog.stata.com/2011/01/05/using-dates-and-times-from-other-software/). With that in mind, it would be nice to have an origin date that can be specified. See also, #3969.

It's a relatively simple thing, and not hard to work around, of course. However, I end up dealing with date formatting on just about every data set I import, and I imagine that lots of others do, too.

Currently, I do something like this:

``` python
import pandas as pd
import datetime

EPOCH1960 = datetime.date(1970, 1, 1) - datetime.date(1960, 1, 1)

data = pd.read_stata('./data.dta')
data['date'] = pd.to_datetime(data['date'],unit='D') - EPOCH1960

```

In R, the `as.Date()` function takes an origin parameter for numeric types (see, [manual](https://stat.ethz.ch/R-manual/R-devel/library/base/html/as.Date.html)). So, in R, the date part would simply be:

``` r
data$date <- as.Date(data$date, origin = '1960-01-01')

```
